Synonyms for Arrant:

adj.

absolute (adjective)
crashing.
all (adjective)
staring, sodding, perfect, double-dyed, pure, everlasting, stark, thoroughgoing, complete, gross, thorough, consummate, utter.
all-encompassing (adjective)
thorough.
bad (adjective)
baneful, irremediable, unsound, vile, poor, wretched, wrong, faulty, ill-suited, inadequate, inadvisable, unpleasant, bad, defective, invalid, black, detrimental, unfavorable, displeasing, inferior, unsatisfactory.
complete (adjective)
out-and-out, thoroughgoing.
egregious (adjective)
egregious.
flagrant (adjective)
unmitigated, glaring, out-and-out, absolute.

n.

unqualified, sound, outright, unreserved, all-out, in depth, flat out, dead, unequivocal, positive, thorough, blooming, resounding, big, unrelieved, out-and-out, absolute, total, limited, unbounded, damned, crashing, sheer, unlimited, plain, flat, downright. rank, egregious, glaring, capital, good, flagrant. arrant (noun)
staring, sodding, perfect, double-dyed, pure, everlasting, stark, unmitigated, complete, gross, thoroughgoing, consummate, utter.

Other synonyms:

crashing, unrelieved. out-and-out, egregious, flagrant, downright. unlimited, flat out, unbounded, all-out. blooming, unequivocal. unqualified, glaring, damned. unreserved, positive, absolute. dead. flat. plain. downright
outright.
Other relevant words:
sheer, unequivocal, good, utter, egregious, glaring, downright, dead, flat, resounding, crashing, everlasting, limited, sound, complete, flagrant, plain, pure, unmitigated, bad, sodding, flat out, capital, rank, unqualified, blooming, thoroughgoing, in depth, damned, double-dyed, all-out, total, staring, big, positive, stark, gross, unrelieved, out-and-out, unlimited, outright, absolute, unreserved, unbounded, consummate, thorough, perfect.