Where does “gwrthiselder” come from?

gwrthiselder (Welsh) comes from Welsh iselder, from Welsh isel, from Middle Welsh isel, from Proto-Celtic *ɸīsselos, from Proto-Celtic ɸīssu, from Proto-Indo-European pedsú, from Proto-Indo-European pṓds, from Proto-Indo-European póds — to walk, to step; to stumble, to fall.

gwrthiselder (Welsh): antidepressant