nonsense, bollocks
[noun]
[de la-rie-koek, <no plural>]

"Lariekoek" is synonymous to "onzin", although it is slightly more informal and a bit slang. Even more informal and a bit vulgar is the synonymous "gelul", which translates to "bullsh*t.

Sometimes you may see the short version: "larie": bull.

Examples:
– "Ik snap niet waarom ik weer 5 kilo meer weeg…" – "Lariekoek! Van lucht word je niet dik!"
("I don’t understand why I’ve gained another 10 pounds…" – "Bollocks! Air doesn’t make you fat!")

– "Gisteren weer een debat in de Tweede Kamer over integratie…ik heb nog nooit zo veel larie gehoord." 
("Another debate in the House of Representatives about integration…I’ve never heard so much bull.")

– "Van boos kijken krijg je rimpels." – "Lariekoek."
("Putting on an angry face causes wrinkles." – "Nonsense." Lit.: "From looking angry, you’ll get wrinkles.")

Expressions:
– "Gelul in de ruimte": Loose talk, nonsense.

Related words:
– Koek: cake [noun] [de koek, de koeken].
– Onzin: nonsense [noun] [de onzin, <no plural>].
– Flauwekul: nonsens [noun] [de faluwekul, <no plural>].
– Gelul: bullsh*t [noun] [het gelul, <no plural>].
– Geouwehoer: rubbish, (load of) crap, bullsh*t [noun] [het geouwehoer, <no plural>].

Example:
– "De kredietcrisis is de schuld van een wereldwijd financieel-militair complex." – "Geouwehoer!"
("The financial crisis is to be blamed on a global financial-military complex." – "Bullsh*t!")

– Kletskoek: waffle, twaddle, rubbish (talk), crap [noun] [de kletskoek, <no plural>].

– Kletspraat: waffle, twaddle, rubbish (talk), crap [noun] [de kletspraat, <no plural>].
– Kletsen: to chat [verb] [kletsen, kletste, gekletst].

Extra:
Apparently, "lariekoek" is also some kind of cake :-). You can find the recipe (in Dutch) here.