According to Pollux 10.187, molgo/s is an oxskin sack in Tarentine dialect. Aristophanes uses it in Knights 963 (web address 1) -- not quoted in the present entry -- and in the passage which is quoted here (fr. 694a Edmonds, 56 Demianczuk, 933 Kassel-Austin) to parallel sayings of the type "I will skin you alive" (since the skin can then be turned into a sack) (LSJ s.v.)
The lexicographer is unaware of this meaning, and instead makes various tenuous connections between molgo/s and a)me/lgw "milk, drain". In this regard, Hesychius' entry (mu1565) is better informed, if not more conclusive: "Aristophanes [sc. uses the term]. Perhaps the word in full might be a)molgo/s. And he who drains money is an a)molgo/s. But some understand as molgou/s evil men, to be thieves to those who drain common property; and a)molgo/s [means the same?]. But others say molgo/s is an oxskin sack. It is also placed on carts."
On the way, the Suda drags in two other words. One is mo/lghs, not molgo/s, glossed as moxqhro/s "evil" by Crates, cited in the scholia to Knights 963. The second word a)molgo/s, also invoked by Hesychius, occurs in nukto\s a)molgw=|, a phrase used in Homer for the dead of night, or for twilight. LSJ s.v. plausibly links it to a)me/lgw, as "milking-time".
The scholia ad loc. also offer the senses "slow or blind" (with various wordings), as well as the first etymology (e)came/lgontas). The gloss "blind" is attributed to the grammarian Phaeinos. The Molgi are stated in the scholia to be referred to in Herodotus, although this does not hold for our accepted text.
See also psi 131.