The money-saving expert repeatedly urged viewers to watch his segment to find out "how to wang your welly for free".
He continued: "If you don't know what that means you're going to have to come back and watch after the break – I'm not going to explain it now."
Stepping in for Piers Morgan and Susanna Reid, temporary hosts Eamonn Holmes and Ranvir Singh were left to quicky step in and clear up what Martin was talking about.
Martin Lewis revealed he would teach viewers "how to wang their wellies for free"
Eamonn had to clarify exactly what Martin said
Folks he did just say 'wang', I just want to make that clear
Eamonn joked: "Folks he did just say 'wang', I just want to make that clear."
But after Martin repeated the cheeky innuendo, Eamonn begged him to be quiet, adding: "I wish you would stop saying that!"
However, shocked viewers thought the 45-year-old made an explicit comment, taking to Twitter to air their confusion.
One posted: "I sure I've just heard Martin Lewis say 'W**k W***y' on GMB? He did have a grin on his face after, and @ranvir01 @EamonnHolmes apologised."
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Confused viewers took to Twitter to see what Martin said Mortgage advice from Martin Lewis Mon, December 19, 2016
Martin Lewis warns that the biggest single personal financial risk in 2017 is that mortgage rates will rise rapidly. Here's the Money Saving Expert's top mortgage tips.
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Mortgage advice from the Money Saving Expert Martin Lewis
Another added: "@MartinSLewis welly w**k Martin?"
A third questioned: "@MartinSLewis What did you just say?"