Dandelli is an Arabic slang word, which comes from the word (Dandell) which means to hang or lay out and Dandelli is the act of popping something out and leaving it to lay freely.
In slang it literally means to lay out your genitals (commonly imagined on a table, for someone to use as a tool!) and its mainly used by LEVANT people.
Another similar word is Dandello which conceptually means the same thing but used in a different context.
A muscular man coming out of a steam bath walking around like a stud.
"dude X" takes a glance at the stud n then turns to his friend and says DANDELLI! or DANDELLO! (in a rough manly voice)
Both of them break into laughing!