rhymes with pester ... so if you pester someone they will get festered, meaning irritated.
fest+er...fest(fist)..hit somebody with your close fist until blood comes out and if it is left untreated it wil become a sore and than a pus..........
Some one keeps pestering you to go faster , faster , faster , faster, faster, faster... (see now ur angry ) ;)
sore pestered becomes festered
fester sounds like blister that oozes pus and causes irritation
fester ->wasted feast giving rotten smell, its look like pus in wound.