Think one explantion of the word to Trap, originates from the greyhound track. The dogs enter the "trap", then they are gone. Other times referred to as being "On the blocks" or "At then starting gate" or even "Doing an EVA " ( extra vehicular activity...space talk!! )
Best Trap ever was to phone in at 4-00pm for a night shift and being told not to bother coming in.... not sure if that counts as a trap though. That as in Air Force days.
Nowadays...Traps almost a thing of the past!!