Okay, several things:
1) I’ll try to upload three times a week from now on: Mondays, Wednesdays and Fridays
2) Two figures of speech for the price of one today!
3) For more information on the diminutive suffix “-(t)je”, see http://en.wiktionary.org/wiki/-tje
4) It’s unclear that the origin of “een uiltje knappen” is, theories are it’s derived from trying to “catch an owlet moth”, or “to take the opportunity to grab the kind of bird”. Since both the birds as well as the moths are nocturnal, it became a euphemism for taking a nap
The word (and phrase) list for today:
mankeert = from “mankeren” (to limp / to fail)
er = there / at that place
bovenkamer = upper room (in this case “brain”)
in de bovenkamer mankeren = being crazy/stupid
want = because
uiltje = little owl (or a member of an owlet moth/nocturnal butterfly family)
knappen = to snap / to crackle
Hope this helps!