indescribable, unbelievable! 
[Dutch phrase of the week]

"Filmen" is the verb derived from the noun "film" and obviously means "to record on film". Unless it is really a film that you are shooting, you will more commonly use "een video opnemen" or "opnemen" ("to record" in general). Apparently this expression is used to say that something is that incredible, it can’t be recorded on film (whatever that means 🙂 ). You can use the expression both for positive and negative astonishment. Or just say: "ongelofelijk!" 🙂

Examples:
– "Moet je die man zien fietsen met die gekke hoed. Niet te filmen…" 
("Look at that man biking over there with that strange hat! Incredible…")

– "Hoe was het congres afgelopen zaterdag?"- "Het was niet te filmen zo slecht… echt, niets was geregeld!" 
("How was the convention last Saturday?" – "It was so bad… it’s indescribable… honestly, nothing had been arranged!")

– "Ik ben dit weekend in Artis geweest."- "Was het leuk?" – "Zeker, vooral die slingerapen, niet te filmen joh hoe die hun staart gebruiken als extra arm!" 
("I went to the Artis Zoo this weekend." – "Did you have a good time?" –  "Absolutely, especially those spider monkeys, it is unbelievable how they use their tail as an additional arm!" The use of the word "joh" is a bit like "ey".)