From toen (“then”) + ter (“at/on the”) + tijd (“time”). The ter is somewhat strange surprising, as it is the contraction of te + the old feminine singular dative definite article der, whereas tijd is a masculine word in modern Dutch. In earlier stages of the language however, the word was feminine. This also explains the compound indertijd and the fixed expression te zijner tijd which both consider tijd to be feminine, versus the younger compound destijds which considers it to be masculine (earliest attestation is in modern Dutch, in 1674).
- at the time, then, during that time