In a shogi context, "slap" is not really a correct translation of "hirate".
But, when the word "hirate" is used with either the verb "utsu" or "tataku" (both which can mean "to hit" or "to beat" or "to tap" depending on the context) then "hirate" can be
translated as "slap" (again depending on the context.
For example, the word "hirate" is often used in Sumo, Mixed Martial Arts or Pro Wrestling commentary to mean an open-handed slap or smack. Often it's used in the form "hirate uchi" which is a combination the
words of "hirate" and "uchi" which is the nominal form of the verb "utsu".
"hirate de utsu" or "hirate de tataku" can be translated as "to slap", "to smack" or something similar.
"hirate de shougi wo sasu" (平手で将棋を指す) means to
play an even (without a handicap) game of shogi.
"hirate de utsu" (平手で打つ) means "to slap someone/something"
"Hirate" is also used as a last (family) name. I've never meant anyone with that name and I'm not sure how common it is.