The combination of Totes (totally) plus McGotes (roughly meaning to agree), yielding a phrase with the meaning to "totally agree" with someone, but in a cooler and stranger way. Also can mean that you agree with an activity, as in "I agree" or "I'm in" as a loose use of totally agree.
Eve: I'm going to go chow down on that apple the snake told us about, are you in?
Adam: Totes McGotes! I'm way down for that! Let's go!
A rhyme negation of the slang totes (meaning totally) this phrase is used as a correction and is followed by the 'better' descriptive (following the pattern of all rhyme -negations)
In the classic tv commercial:
Girl 1: "He's totes adorbs"
Girl 2: "totes magotes, he's cray-cray amaze"
(this follows the standard pattern for rhyming negation:
Friend 1: "What should I wear? It has to be conservative"
Friend 2: "conservative shmervative, go wild!")
note: In the full commercial, the follow up comment of "totes magotes" is used incorrectly.