Abarim Publications' online Biblical Hebrew Dictionary
מצה מצץ
The two roots מצה (masa) and מצץ (masas) are formally two but might very well be considered one and the same:
מצה
The verb מצה (masa) means to drain out. It occurs all over the Semitic language spectrum and is used seven times in the Bible: Gideon drained dew water from the fleece (Judges 6:38) and the Psalmist speaks of "waters of abundance" draining from scoffers (Psalm 73:10). Several times a certain cup of calamities is drained (Psalm 75:8, Isaiah 51:17, Ezekiel 23:34). Twice this verb is used to describe the draining of blood of a decapitated bird carcass from the altar (Leviticus 1:15, 5:9).
מצץ
The curious verb מצץ (masas) occurs only in Isaiah 66:11, which makes it hard to interpret. But since it describes what an infant does to a breast, scholars are pretty confident in considering it a by-form of the verb מצה (masa), meaning to drain (see below). Our verb מצץ (masas) has only one derivative, namely the noun מצה (massa, which is spelled the same as the other verb). It literally means "something drained" and denotes unleavened bread:
Unleavened bread
The noun מצה (massa) literally describes something "drained/illiquid" and specifically denotes unleavened bread. Leaven is a fungus that creates bubbles in bread (and beer) and unleavened bread is bread without spirit; bread without oomph. The word for leaven, שאר (se'or), literally means remnant, and anything unleavened was either made in such haste that the residual starter batch of leaven had had no time to spread through the rest of the dough, or otherwise, the baker had had no access to a properly cultivated culture (1 Samuel 28:24).
Unleavened bread (מצה, massa) came in the form of לחם (lehem), which is general bread, חלה (halla), possibly a donut or pretzel, and רקיק (raqiq), a thin wafer (all three are mentioned in Exodus 29:2) or עגה ('uga), a circular and probably flat, pancake-like bread (Exodus 12:39).
But there is quite a symbolic load attached to this unleavened bread. Anything leavened was called חמץ (hames), which is probably closely related to the verb חמץ (hamas), meaning to violate or push into the wrong direction. The word for bread is לחם (lehem), whereas the highly similar verb לחם (laham) means to fight or do battle. Our noun מצה (massa), in turn, is closely similar to the noun מצוה (miswa), meaning commandment. In fact, in Genesis 26:5 it is not clear whether the Lord praises Abraham for keeping his commandments or his unleavened breads (plural: מצות). And the rare equivalent of our plural (מצת; Exodus 12:18), is strikingly similar to a rare form of the verb מצא (masa'), meaning to find (מצתי; "I have found", Numbers 11:11).
The first time מצה (massa) is mentioned is in Genesis 19:3, where Lot bakes it for his angelic guests, and probably for a very good reason. Leavened bread was considered better than unleavened (that's why it was invented) but Lot made his guests a full-fledged משתה (mishteh), that is a banquet with all the fixings, making it clear that the absence of leavened bread was not due to any lack of funding. Leaven has to be carefully cultivated and is passed on from one batch of bread to the next, and there appears to be a strong intuitive connection between leaven and the many winds of gossip and fashion that blow a nation. By serving מצה (massa), the host appears to ostentatiously distantiate himself from the "leaven" of the society around him.
And that explains why the last meal of the Hebrews in Egypt consisted in part of unleavened bread (Exodus 12:15-20), and even though just prior to their departure they relieved their Egyptian neighbors of their silver, gold, clothes, fellow slaves and huge amounts of flocks, herds and livestock (Exodus 12:35-38), they were not to take any of Egypt's leaven with them. Israel had to make a fresh start, leaven-wise.
Unleavened bread came to play a huge part in Israel's ritualistic prescriptions (Exodus 29:2, Leviticus 2:4-5, 7:12, 8:2, 10:12, Numbers 6:15-19) and particularly the Feast of Unleavened Bread (חג המצות, hag hamassot; Exodus 23:15, 34:18, Leviticus 23:6, Deuteronomy 16:16, 2 Chronicles 8:13, 30:13, 30:21, 35:17), which appears to have been a separate feast that coincided with that of Passover (like celebrating two people's birthdays in one party).
Additional note
The above words are part of a pretty impressive symbolic super-structure. The verb מצה (masa) means to drain but the verb מצא (masa') means to find. That is significant because the verb נצה (nasa I) means to fly and is also spelled as נצא (nasa'). The identical verb נצה (nasa II) means to struggle, whereas its derived noun מצה (masa, spelled the same as the first verse of our list), means strife.
The identical verb נצה (nasa III) means to fall into ruins. The noun מוץ (mos) means chaff, and the noun מוצא (mosa') denotes a place or act of going out. The origin of the former is missing in action (we formally don't know from which verb it derives) but the latter comes from the verb יצא (yasa') meaning to go out. There is no verb יצה but there is a noun צוא (se'a) that means filth, and a verb צוה (sawa), meaning to command, from whence comes the noun מצוה (miswa), meaning command, as mentioned above.