The act of blowing marijuana smoke in the direction of a distracted friend to let them know it's their turn to hit the blunt.
Johnny, who was already in a daze from excessive smoking, got a whiff of the smoke signal, and then knew it was his turn to hit the blunt.
To smoke signal is to stand outside a fellow tokers window from a distance and with a skilled puff, shooting a sliver of smoke, taking in account wind, through an open window. A trained toker will instantly recognize the smell of the sweet cheeba and know they are being summoned.
I stood outside Dilbert's house and smoke signal'd. Within 30 seconds Dilbert was beside me hittin some cannabis.