From grate (“(obsolete) serving to gratify, agreeable, pleasing; grateful, thankful”) + -ful (suffix forming adjectives with the sense of tending to have or thoroughly having a quality). Grate is a learned borrowing from Latin grātus (“agreeable, pleasing; beloved, dear; grateful, thankful”), ultimately from Proto-Indo-European *gʷerH- (“to express approval, praise; to elevate”).