Being someone’s secret lover.
I saw Jack sneak into Sally’s house last night, he’s her undercover lover.
undercover lover: define #2
Non-alcoholic drink disguised as a cocktail; commonly drank by pregnant women not yet ready to make the news public. Ex: soda water with a lime in a tall glass that appears to be a vodka tonic.
I discreetly ordered an undercover lover from the bartender so my girlfriends wouldn’t know I’m not drinking.