"Lousy" also has a definition of being infested with lice (the singular form of which is louse); according to the Etymology Dictionary this may be the original

"Lousy" also has a definition of being infested with lice (the singular form of which is louse); according to the Etymology Dictionary this may be the original meaning (https://www.etymonline.com/word/lousy https://www.etymonline.com/word/lousy) From there, it seems to have developed as an American slang to describe "infestations" of other kinds.