toot · noun, verb1
Probably onomatopoeic in origin. Compare Dutch toeteren (“to blow a horn”), German tuten, Swedish tuta, Danish tude. Noun etymology 1, noun sense 7 ("Mastodon post") and verb etymology 1, verb sense 10 ("to post on Mastodon") are influenced by tweet (“Twitter post”).
toot · noun2
Perhaps a contraction of toilet.
toot · noun3
An alteration of tot, borrowed from Scots.
Etymology from Wiktionary, CC BY-SA 4.0.