different between backsies vs backsides

English

back +? -sies

backsies

backsies (uncountable)

(childish) The act of taking back or going back on one's word, promise, or gift.
- 1987, Carolyn W. Field, Jaqueline Shachter Weiss, Values in selected children's books of fiction and fantasy
  Frances is the loser when she agrees to no "backsies" or refunds as she buys Thelma's plastic tea dishes with money saved for a china set.
(childish) A reciprocal action or consequence, such as immediately tagging the player who has tagged one in a game of tag.
The act of joining a queue behind a friend rather than at the end of the queue.

backsides

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