Deriving lactuca from lac

Question

The word lactuca refers to lettuce, and Lactuca sativa is the scientific name. Some of the plants in this genus seem to contain some kind of milky liquid which must be the reason for deriving the word from milk. (I do not know what kind of lettuce the Romans had. Intensively cultivated plants have evolved quite a bit over the last two millennia, so it is hard to guess how milky their lettuce was.)

How exactly is lactuca derived from lac? I am not familiar with this suffix -uca. Is this a regular derivative? Are there other examples? What does the suffix mean — does the plant look taste like milk, contain milk, look like milk, does it help milk a cow, or something else? The last question is intended from a linguistic point of view, not a biological one.

Answer 1

The suffix -ucus was added to the stem of lactare, just like it happened in caducus from cadere and in manducus from mandere. Fiducia too should come from an unattested *fiducus (thanks @TKR).

According to Ernout (Dictionnaire etymologique de la langue latine) the original word was a feminine adjective, with herba implied. It meant milky grass, and it came from the milky liquid inside. He seems to imply that the suffix should be more or less equivalent to a present participle (lactens, cadens), which makes sense at least looking at their meaning.

Szmerenyi (Greek γἀλα and the Indo-European term for „milk“) says:

Lactuca can hardly be explained as bearing the same relation to lacto as caducus to cado, since lact-are is different from cad-ere; if Ernout is right in regarding late lactare as based on earlier lactans, then at the time of Lucilius or before, when only lactans existed, it was quite impossible to produce a new lactuca on the pattern of cad-ens: cad-ucus. Since, on the other hand, γαλακτουχος is attested in Greek, it is reasonable to infer that lactuca is a Latinization of γαλακτουχη, just as *tartaruca (italian tartaruga) is from Greek tartarouxos.

Leumann (Lateinische Grammatik, vol. 1, par. 304) simply reports the suffix as a "deverbative adjective", because "no big functional group [funktionell bestimmten gruppen] can be found for adjectives and nouns coming from -co- and -quo-". He follows Szemerenyi, KZ 75, 178 (Greek γἀλα and the Indo-European term for „milk“) in saying that lactuca is a substantive coming from Greek γαλακτουχος, which Leumann translates as "containing milk".

Walde (Lateinisches etymologisches Woerterbuch) only says: "from milk, because of the milky juice".