Where does “косо-” come from?

косо- (Russian) comes from Russian -кос, from Russian коси́ть, from Proto-Slavic kǫsiti — to mow, to reap.

Ancestry of “косо-”, step by step

StepLanguageWordMeaning
1Russian-кос—
2Russianкоси́тьto twist, to slant; to twist, to slant, to...
3Proto-Slavickǫsitito mow, to reap
Every word from Proto-Slavic kǫsiti →Every word from Russian коси́ть →Every word from Russian -кос →