걸래는 빨아도 걸래다 have two meanings? Since
빨다 can be used both for "wash" and for "suck", does this also mean something about soaking up water as well as the more obvious meaning to wash in water?
Does 걸래는 빨아도 걸래다 have two meanings?
Literally, it means 'the rag is still rag after wash'.
But more often, this is idiom states that 'You cannot undo what you have done'.
Also sometimes 걸레 refers to the women (or men as well, but less frequently) who is sexually active, somewhat like 'bitch' in english speaking countries.
Therefore this idiom sometimes used to offend someone who are considered to be sexually active.
I would say this is not very polite to say in public.
does this also mean something about soaking up water as well as the more obvious meaning to wash in water?
No. Here the semantic meaning of 빨다 is 'wash'.
It literally means "A rag is still a rag, even if you wash it".
빨다 here means "to wash (clothes)".
Figuratively, it means "No matter how hard you try to improve a thing in a certain way, its innate nature will not change".
However, 걸레 has another slang meaning, which translates to "slut". So the innocent looking sentence could also be interpreted as: "A slut is a slut even though it's been washed". Which is a totally inappropriate thing to say in most cases.