private joke, secret amusement
[noun]
[het bin-nen-pret-je, de bin-nen-pret-jes]

"Binnenpretje" is composed of "binnen" (inside) and "pretje": diminutive of "pret" (fun, amusement) and means something like "fun on the inside" or: "private joke". When somebody is suddenly smiling or chuckling to himself without any context, you may assume he or she has a "binnenpretje". The noun is "binnenpret" but we typically use the diminutive.

Common usage: "Waarom lach je?" – "O niks, gewoon, binnenpretje!"

Examples:
– "Hij dacht dat ik hem uitlachte, maar ik zweer het je, ik had gewoon een binnenpretje." 
("He thought I was making fun of him, but honestly, I just had to chuckle to myself." Lit. "I swear it to you".)

– "Sander kijkt altijd zo serieus, heb jij die gast ooit betrapt op een binnenpretje?" 
("Sander always looks so serious, have you ever caught that bloke/dude chuckling to himself?")

– "Met Irene valt echt niet samen te werken, de hele tijd dagdromen en binnenpretjes!" 
("Irene is impossible to work with, all the time it's daydreaming and private jokes!")

Related words:
– Binnen: inside, indoors [adverb].
– Pret: fun, pleasure, amusement [noun] [de pret, <no plural>].
– Prettig: pleasant, enjoyable, nice [adverb/adjective].
– Gniffelen: to chuckle [verb] [gniffelde, gegniffeld].

Extra:
A popular brand for liquorice, wine gums etc. in the Netherlands is "Red Band". Their slogan is well known amongst children and adults: "Red Band, als je voor pret bent!" I mean, who is against fun??