1 - n. the literal act of tucking ones penis back between their legs to resemble a female crotch.
2 - n. A person who engages in this activity whether voluntarily or by reflex. Used in a derogatory fashion to indicate somebody who is a coward, i.e. they are so scared that their penis retracts into their body. To pull a dicktuck would be to act like a braggart, claiming you are going to perform an action (typically a fight) and then backing out at the last minute. To back down from a shocking claim in a dishonourable manner.
Related to 'pussying out', although the dicktuck infers that the tuck-ee was acting like a big man or asshole prior to the cowardly withdrawl
Guy at the bar said he was gonna kick my ass when I left, but he pulled a dicktuck when he saw me stand up
I see you edited your post after you were getting pwn3d. Nice dicktuck, Charlie.