Webster's Revised Unabridged Dictionary · 1913

Slap

Slap , noun

[Old English slappe; akin to LG. slappe, German schlappe; probably of imitative origin.]

A blow, esp. one given with the open hand, or with something broad.

Slap , transitive verb

To strike with the open hand, or with something broad.

Slap , adverb

[Compare LG. slap, German schlapp. See Slap, n.]

With a sudden and violent blow; hence, quickly; instantly; directly. [Colloquial]
The railroad cars drive slap into the city. — Thackeray