This could either I mean 'I guess' or 'Instagram' depending on the conversation.

It's the sweat that accumilates under big boobs

When someone texts you something with, "Ig", you may think they are mentally ill, or they didn't finish saying "Igloo" for some odd reason. Actually, they mean, "I guess", because nobody has time to say, "I guess".

Shortened way of saying"I guess".

Ig can also mean "I guess"

This word is short for I guess and in some rare cases instagram.