irksome (adj.)

early 15c., irken, "to trouble (someone), disturb, hinder, annoy;" earlier "be lax, slow, or unwilling (in doing something); be displeased or discontented" (early 14c.); "be weary of, be disgusted with" (c. 1400); of uncertain origin, perhaps from Old Norse yrka "to work" (see work (v.)).

Watkins suggests it is related to Old Norse yrkja "work." Middle High German erken "to disgust" also has been suggested. A Middle English adjective, irk, meaning "weary, tired, bored; distressed, troubled; troublesome, annoying," is attested from c. 1300 in Northern and Midlands writing; it is sometimes said to be from the verb, but it is older, and Middle English Compendium says this is probably Celtic, and compares Old Irish arcoat "he injures," erchoat "harm, injury."

word-forming element used in making adjectives from nouns or adjectives (and sometimes verbs) and meaning "tending to; causing; to a considerable degree," from Old English -sum, identical with some, from PIE root *sem- (1) "one; as one, together with." Cognate with Old Frisian -sum, German -sam, Old Norse -samr; also related to same.

"It usually indicates the possession of a considerable degree of the quality named: as mettlesome, full of mettle or spirit; gladsome, very glad or joyous" [Century Dictionary]. It is also, disguised, the ending in buxom. For the -some used with numbers (twosome, foursome, etc.), see -some (2).