Charles Ellicott Commentary Matthew 26:73

"And after a little while they that stood by came and said to Peter, Of a truth thou also art [one] of them; for thy speech maketh thee known." — Matthew 26:73 (ASV)

Your speech betrays you — The Galilean patois was probably more pronounced when he spoke under the influence of strong excitement. Its chief feature was said to be a confused, thick pronunciation of the guttural letters of the Hebrew alphabet, making them indistinguishable from each other and causing the sound Sh to be changed into Th.

This partial recognition implied by the remark, perhaps combined with a sense of shame that his provincial accent was attracting notice, led to the more vehement denial that followed.