greetings

Hello

greeting noun
3 greetings humorous an old-fashioned way of saying hello to someone
(from MacMillan English Dictionary)

Why and in what way is the expression 'greetings' humorous?

Thank you

I don't know why they say "greetings" is humorous.

Our dictionary has this under greeting:
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Any word that is formal might also be used homorously, if it is said with exaggerated formality, or in a very informal setting. However, in my experience, 'greetings' is also used as straight-forward expression of welcome.

However, in my experience, 'greetings' is also used as straight-forward expression of welcome.

In the USA, maybe. I note that the definition is taken from a British dictionary, and I would be inclined to agree that the use of "Greetings" as a word of welcome to visitors is associated with an arch, hearty naffness: I imagine a stereotypical user as being a bearded, sandals-with-socks-wearing middle aged male with tiresome social opinions ... I'm sure you get the picture.

I share DocPenfro's view on "Greetings".
Other images it brings to mind include some from badly-made science fiction movies in which an improbable alien alights from a saucer-shaped object, raises a tentacle, and voices something akin to "Greetings Earthman. We come in peace."