Mhairi Black, who represents Paisley and Renfrewshire, had criticised the proposed withdrawal of housing benefits for 18 to 21 year olds shortly before the jibe.
Speaking in the Commons on Tuesday, the 22-year-old argued: “The Scottish government cannot reverse the cuts, or provide an exemption for Scotland.
“Does the minister agree that it is simply ridiculous that young people should suffer purely because this Government is obsessed with imposing austerity?”
Mhairi Black appeared to mouth "you talk s***e" to Caroline Nokes
You talk s***e, hen
Tory MP Caroline Nokes, responded: “The Scottish government already has a wide range of powers which would enable them to alleviate the changes being proposed here.”
However, as the Department for Work and Pensions ministers continued, the camera cut back to the SNP MP, who shook her head while appearing to mutter the expletive phrase.
The clip, which was tweeted by Ms Black, has since sent some social media users into meltdown as they speculated what was actually mouthed.
One woman wrote: “No much of a lip-reader but definitely mouthing, “you talk s***e, hen”.
Mhairi Black become riled during a debate about housing benefit cuts for 18 to 21 year olds
Yet more cuts that Scotland didn't vote for and no real answers from the Minister. pic.twitter.com/CPhYFtYfcU
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— Mhairi Black MP (@MhairiBlack) March 7, 2017
A second added: “Lip reader here, it’s ‘dinnae talk s**** hen’”.
While a third quipped: “It’s certainly not ‘I do believe you’re telling some fibs minister’.”
At just 22 years old, Ms Black is the youngest member of the House of Commons.
She entered the chamber in May 2015 when she was first elected aged 20.