Do not make yourselves disgusting through all the swarming creatures, and do not make yourself impure through them, venitmetem (ונטמתם) through them. (Vayikra 11:43)
What does “venitmetem” mean? The most obvious explanation is that it means “and you shall become impure”, being derived from the word “tamei”. That fits well with the theme of impurity which is a central aspect of the kosher-animal chapter, though it does seem redundant, since impurity was already mentioned in the previous phrase. (The problem with this explanation is that “tamei” has the letter alef in it, and “venitmetem” does not. However, there are cases in Tanach where an alef disappears from the spelling of a word, since it is not pronounced. This could be another such case.)
Ibn Ezra on the verse cites the above explanation, but then brings an alternative. “Some say they [tamei, venitmetem] are two different roots, as in [נטמינו בעיניכם], its meaning being, like a person who does not have wisdom.” If so, then the verse should be translated: “...and do not make yourself impure through them, and become foolish through them.”
This grammatical understanding may be the “pshat” basis for the idea of “timtum halev”. As for what the idea actually means, well, that's beyond the scope of this post. But you can find an interesting discussion here.