köken: Slovak rýdzi (“pure; (historically) reddish”, adjective) + -ik, the motivation is the reddish colour of the cap and especially the milk of the mushroom.
Inherited from Old Polish rydz. By surface analysis, back-formation from rydzy. Cognate with Bulgarian рижика (rižika), Russian ры́жик (rýžik). Compare Spanish robellón (distantly cognate via Proto-Indo-European *h₁rewdʰ-).