op de kop tikken

Literally, “to tap on the head”. Three explanations of its origin are given.

The first is from hunting, where a trapped rabbit or other animal would get a knock on the head to finish it off (where “knocking on the head” would mean one is successful in their hunt and does not go home empty-handed).
The second is from carpentry, where hitting a nail on the head means being successful, as one would be when acquiring something desired at a low price.
The third explanation is that one would indicate what they wanted to buy by tapping the object in question, whereby the act of tapping the object would become synonymous with buying it.

op de kop tikken

1. (idiomatic) to buy something (at a favorable price)

   Ik heb op de rommelmarkt een leuke vaas op de kop getikt.

   I bought a nice vase at the flea market.

• Onze Taal, iets op de kop tikken