Sheepish means embarrassed at having been caught out. A sheepish grin, look sheepish, smile sheepishly.
The embarrassment is mild and slightly comic, which is why the word usually goes with a grin. It implies you know you were in the wrong. The image is of a sheep — meek and easily driven — rather than of any particular sheepish behaviour.
1adjectivesheepishEmbarrassed at having been caught out.
“He gave a sheepish smile when his mum found the chocolate wrappers under his bed.”
sheepish is a quality of manner.
Sheep plus -ish, from the meek and easily driven look of the animal.
Middle English
schepische
English, today
sheepish
From Middle English schepische, shepisse, shepisshe; equivalent to sheep + ish.
Etymology from Wiktionary, CC BY-SA