
ETYMOLOGY
either from a) high (adj.) + -ful;
or perhaps from b) the same word as eyeful (arousing or inspiring awe, fear, or dread; terrible, dreadful)
EXAMPLE
“…So þat he was riche him sulf · & þat lond pouere al out
Sturne he was þoru out al · & heiuol & prout
Suiþe þikke mon he was · & of grete strengþe…”
From: The Metrical Chronicle of Robert of Gloucester, c1325-c1425