Etymology

(nota bene: in this proverb, the word money is in its plural form.)

From peningar, the nominative plural of peningur (meaning "money") + vaxa which is third person plural (meaning "[they] grow") + ekki (meaning "not") + á (meaning "on") + trjám (meaning "[on] trees"), the dative plural of tré (meaning "tree"). Literally meaning "money [they] grow not on trees". See also the Swedish pengar växer inte på träd.