Green’s Dictionary of Slang

low five n.

[the opposite of a high five n.1 ]

(US black) a palm-/hand-slapping ritual, with the hands held low rather than the usual high five n.1

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[US]Time 3 May 80: Dodger Leftfielder Dusty Baker thought the San Diego hand-slapping (high fives, low fives) a little elaborate for April [HDAS].
Phila. Weekly 🌐 4 June Now he greeted Turk with a lowfive and a robust, ‘My man, Little A.I.!’.

In phrases

give oneself a low five (v.)

to masturbate.

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[US]quinnelk T. Rex’s Guide to Life 🌐 Okay, since people don’t want to actually say the m-word and the chicken and monkey phrases have been used to death on MTV, I thought it would be my duty to provide you with a bevy of other useful terminology that may be helpful in this area: [...] giving yourself a low five.