The letters I, S, V, G, A, B, E, A and N came up and the contestants were given 30 seconds to come up with the longest words they could.
She was left in stitches when player Ellen Guy put together the word “vaginas”, leading the audience to break out into naughty tittering.
The 31-year-old mathematical genius could not stop laughing as she tried to suppress a smile with not much luck.
Singer Jimmy Osmond, who was this week’s guest in Dictionary Corner, could be seen laughing as he said: “I think that about does it! We have saving – not nearly interesting enough.”
Rachel tried not to laugh again as the singer smirked about the word.
However, host Nick Hewer tried to move things on as he briskly said: “I think we’re happy enough, we’ll move on now.”
The former Apprentice star look less impressed at the immature laughter all round as he continued.
Rachel Riley could not stop laughing on Countdown
Contestant Ellen put together a rather rude word on Countdown
Despite getting the conundrum in the final round, sales administrator Ellen could not catch up with university student James Slater.
The final score was 78 to James and 61 to Ellen, which means the LSE student will be returning but sadly it was farewell Ellen from New Malden.
This is not the first time a rude word has come up on the Channel 4 afternoon quiz.
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The contestants battle it out on Coutndown Countdown's rudest words Thu, April 20, 2017
Countdown: 14 times the show was rude and sometimes unbelievably filthy.
Channel 4 1 of 16
This is the hilarious moment Countdown brainbox Rachel Riley was forced to spell out the word BUMHOLE
During a letters round last month, one contestant put together six-letter word "erotic", while another said: “Coming."
Rachel attempted to stop giggling at the crude word combination.
Countdown airs weekdays on Channel 4 at 2.10pm.