A laugh is actually just "hi", although sometimes it would be repeated if conditions were noisy. Then it would be "hi hi" with a space in between, not all sent as one word.
So why was "hi" used to indicate a laugh in Morse in the first place? There's no obvious connection with either the word "hi" or the individual letters "h i".
The reason is that the meaning here is not in the actual letters or word, it's the sound of "hi" in Morse:
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It just sounds funny to hear that. A bit reminiscent of the rhythm of the old phrase "shave and a haircut, two bits" but more concise.
Oddly, many hams to this day also use "hi" or "hi hi" in voice contacts instead of laughing. I guess that's not too different from saying "lol" out loud!
"es" is another interesting abbreviation. This of course is the Spanish word for "and", but hams always use "es" instead of "and" in Morse. It's one letter shorter, but that's not the real reason. Here's "and":
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