you said your dd had tried to slap your other child, so she knew what slapping was? is it possible she tried to slap someone else and the nanny spoke to her about it - expanded her vocabulary a bit?
i only ask - my dc's have come up with some really strange 'so and so hit me/ smacked me' stuff which seems plausible enough at first, and turns out out to be a total misunderstanding.
once we spent two weeks trying to work out when 'cousin ben hit me' for 2yo dd2. it turns out that the 'cousin ben' she was referring to wasn't cousin ben at all, but ben next door (presumably she was only calling him 'cousin ben because she was used to hearing it), and 'me' was 'mia' his sister, and she had been watching them through the fence. cousin ben hit me indeed. it went on for weeks, and we were genuinely worried.
i never know where they pick up language or anything. 2yo dd1 picked up an action man bayonet belonging to an older friend's ds and used it to 'skewer' 6mo ds1 in the ear, perforating his eardrum. my friend who witnessed the incident said she just picked it up off the floor, stuck it in her brother's ear, and twisted it, saying 'skewer' really clearly. she wasn't being malicious, just experimenting...
small children baffle me.
i've always had good relationships with our nannys though - but i think if i had mentioned that one of the dcs had accused them of slapping them they'd be fairly defensive. 