Young Thug -- Joins Mile High Club ... With Sizzurp

Rapper Young Thug apparently hasn't heard airports have been crawling with cops for about 13 years ... because he rolled out of LAX Saturday with a double cup ... of sizzurp.

YT -- an up and coming Atlanta rapper with lots of buzz -- didn't specifically say sizzurp, but he laughed when we asked and said we knew exactly what it was ... so there you go.

As you know ... the signature of sizzurp is the double cup.