means A letter or group of letters added to the end of a word to change its meaning or grammatical function, like '-ness,' '-ing,' or '-ly.'

from From Latin suffixum, the neuter past participle of suffigere, 'to fasten underneath or attach.' That breaks into sub- ('under, after') plus figere ('to fix, fasten') — the same figere that anchors words like 'fix' and 'crucify.' So a suffix is literally something fastened on after, tacked to the tail end of a word and pinned firmly in place.