hrj

All items have been vetted for currency in the 17th century via citations in the Geiriadur Prifysgol Cymru. Note that some of the grammatical forms are gender-specific and only the male versions are given (due to the wording of the question).

adyn - “wretch, scoundrel”
bawddyn - “vile wretch”
budr - “dirty, filthy, nasty” (as adj. or n.)
budryn - “wretch, knave”
casddyn - “hateful person, hated one”; also related “cas” = “hated person, enemy” but this appears to be obsolete in this sense by the 17th c.
cenau - “rascal”, but primarily “whelp, puppy” and even the “rascal” sense seems to be more affectionate than derogatory
cnaf - “rascal, knave”, a borrowing of English “knave”
croesan - “fool, buffoon”, earlier use could also mean “jester” but by the 17th c. it seems to have been entirely pejorative
diog - “lazy, sluggish” (as adj. or n.)
diogyn, diogwas - “idler, sluggard, lazy person”
drwg - “bad, evil, wicked” (as adj.)
drygwr - “bad man, evil doer, scoundrel”, can also be a reference to the Devil
dwl - “dull, stupid, foolish” (as adj. or n.)
ffo^l - “foolish, silly” (as adj.), “fool, clown” (as n.)
gwalch - “rascal”, originally “hawk, falcon” and with an extended sense of “hero, nobleman” in the 14-15th c., but by the 17th c. the pejorative sense comes in; possible too ambiguous as the primary associations are positive
gwirion - “simpleton, fool” more in a “mentally impaired” sense than in a “stupid” sense
hwrswn - “whoreson” (borrowed from English)
lleidr - “thief”
llwfr - “coward” (as adj. or n.)
meddwyn - “drunkard”
musgrell - “feeble, clumsy, slovenly”
ofer - “worthless, useless”; oferddyn, oferwr - “waster, good-for-nothing” (but also “minstrel”)
penbwl - “blockhead, stupid”
pendew - “stupid”, lit. “thick-head” (as adj.)
segur - “idle, lazy” (as adj.)
truan - “wretch, unfortunate person” (more in pity than disgust)
twyllwr - “deceiver, cheat, fraud, imposter”
ynfyd - “foolish, mad, insane” (as adj.)