Gosh, I cannot believe you
You went on three dates in the same night? GICNBY!
Psh, that was a slow night for me.
|GICBTS||Gosh, I can't believe that stuff|
|I can't even||I don't know what to say|
|Whoa||Expression of surprise|
|NFW||No freakin' way|
|2G2BT||Too good to be true|
Someone who sends you GICNBY (gosh, I cannot believe you) is tired of your nonsense. You're most likely to receive this acronym in messages sent by your friends, family members, or co-workers after you do something absurd and/or terrible again.
More rarely, people might send GICNBY when they think you're lying about something. If you're being serious and truthful, you can respond to GICNBY with no cap or srsly.