Today's 'I' word has a hyphen (so I suppose I'm cheating a bit!) but I do like it:
ill-willie or ull-wullie
This word can be spelled 'ill-willy' or 'ill-wully' too and can mean bad-tempered or mean.
The Dictionary of the Scots Language (DSL) has a few examples of its use dating mostly from the 1700s onwards:
J Kelly's Complete Collection of Scottish Proverbs (1721) has the following:
An ill willy Cow should have short Horns.
and a later example from William Shelley's Flowers by the Wayside (1868):
Let dour ill-willie sit and fret, The while I lilt the ither sang.
It's another one of these words that is still in use, but not seen (or heard) all that often. It seems probable that it is derived simply from 'ill-willed' - but I like the sound of 'ull-wullie' better!