to stray, commit adultery, be unfaithful, to
cheat  
[verb]
[vreemd-gaan, ging vreemd, i. vreemd-ge-gaan]

"Vreemdgaan" is composed of "vreemd" and "gaan", which respectively translate to "strange" and "to go". "To go strange" is of course a somewhat literal description of engaging in sexual intercourse with someone other than one's regular partner…

Examples:
– "1 op de 4 mensen gaat vreemd."- "Ja, doei!" – "Nee, echt!" 
("1 out of 4 people commits adultery." – "Yeah right!" – "No, really!")

– "Er zit geen vooruitgang in Franks gedrag. Ik voorspel je dat hij binnen een week weer vreemdgaat." 
("There's no progress in Frank's behaviour. I'm telling you, he will be unfaithful again within a week.")

– "Staatssecretaris Jack de Vries is afgetreden nadat bekend werd dat hij vreemdging met zijn assistente." 
("State Secretary Jack de Vries resigned from office after it became public that he committed adultery with his assistant.")

Expressions:
– "Overspel plegen": to commit adultery. Synonymous to "vreemdgaan".
– "Buitenechtelijke relatie": extramarital relationship.
– "Een scheve schaats rijden": (lit.: to ride a crooked skate) to have an affair.
– "Buiten de deur neuken": (lit.: to f*ck outside the door) <vulgar> to have a sexual, extramarital relationship.

Related words:
– Affaire: affair [noun] [de affaire, de affaires].
– Bedriegen: to cheat (on) [verb] [bedriegen, bedroog, h. bedrogen].

Example:
– "Als ik wil, kan ik je zo bedriegen. Ik heb vele aanbidders." – "Schat, wil je me iets duidelijk maken?"
("If I want, I can cheat on you just like that. I have many worshippers." – "Honey, do you want to make something clear to me?")

– Overspel: adultery [noun] [het overspel, <no plural>].
– Schuinsmarcheerder: adulterer (lit.: he who marches askew [noun] [de
schuinsmarcheerder, de schuinsmarcheerders].
– Slippertje: affair [noun] [het slippertje, de slippertjes].
– Vrijen: to make love [verb] [vrijen, vree/vrijde, gevreeën/gevrijd].
– Verhouding: affair, relationship [noun] [de verhouding, de verhoudingen].