1, adj. Cute, funny, and dumb in a endearing way.
2, noun. Foolish, dumb or ridiculous.
3, adj. A description that seems to defy grammar rules, that you also have to hear in the right context to understand, therefore has no good
definition. It's somewhat similar to the first
definition.
4. The first
definition, but in an ironic way. Something that isn't silly being called that due to the fact it is well liked or just labeled as such.
It came from the old English "Seely", a mispronunciation of Happy.