I understand louse being singular for lice; however, I'm uncertain as to why louse around means to idle and louse up means to ruin. Any ideas?
A louse is a common derogatory term to describe a person who acts like a parasite, a pest, or someone extremely unpleasant.
Merriam-Webster offers this example: Her ex-husband is a real louse.
D.C states: Slang meaning "obnoxious person" is from 1630s; to louse up "ruin, botch" first attested 1934.
The earliest instance of the phrase louse around I found in Google books was American Dialect Society, 1927
v. phr. To loiter about aimlessly. Also, Laze around.
Which suggests that the term was not as derogatory as it is today. The better-known idiom, to lounge around, is but another way of saying to laze around. The following excerpt confirms this meaning.
From 1931 Shoe the Wild Mare by Gene Fowler
Nowadays however, the idiom expresses contempt and suggests a parasitic approach to life. Wikitionary says
- (UK) To slack off; be lazy; be a "parasite" to someone/something.
The verb louse has two completely opposite meanings, to remove lice or to fill with lice (this apparent contradiction is not that rare, there are other verbs formed from nouns and meaning both adding or removing whatever the noun is).
Filling with lice generally makes something bad. Hence figuratively, any act that spoils or ruins is lousing or lousing-up.
"Louse around" sounds like an eggcorn for "lounge around", though since it isn't considered particularly productive, it could be as much a portmanteau of "lounge around" with louse in the sense just considered.