From the same stem as rets (“thin, rare, scarce”) (q.v.), with adjectival derivation: *ret-snas > resns. Maybe originally used of trees: rare, sparse trees, separate from others, tend to be thick; this sense could then be generalized to other tree-like objects. A different opinion is that resns is related to Old High German risi (“giant”), Old Church Slavonic редъ (redŭ, “food”), and perhaps, via metathesis, with Latvian vērsis (“ox”); but the Lithuanian cognate suggests that the original meaning of this word was “stout,” “strongly built,” not “well fed.” Cognates include Lithuanian rẽsnas (“squat, strong, stout”).