Stick one's foot into one's mouth?

[Guest]

"Didn't I just say that Turkel manages to stick his foot into his mouth in almost every article he writes? I have lost count of how many times he has done it in this article,..."

(Read from http://www.theskepticalreview.com/olivet/humpty-3.html)

What does "stick one's foot into one's mouth" mean?

[Conchita]

‘To stick (or put) your foot in your mouth’ or
‘to put your foot in it’ means

‘to make a tactless remark, or do or say something embarrassing or get yourself into trouble’ (which can happen if you don’t think carefully before speaking or doing something!)

Synonyms: blunder, err, goof, slip up, speak out of turn, be indiscreet, be tactless