I came across the phrase やめときます in alc.co.jp, which is apparently translated to things like "I'll pass" or "I'd better not."
It didn't appear as a stand-alone word in the dictionaries I checked, so I'm wondering if it's a contracted form of two other words.
Just for the sake of making a guess...I'll guess that it's 止め (imperative form of 止む) and 説く.
As for grammatical terms, I've always thought of it as the テ形 (te-form), but I have a few books which use different terms. In this case, the テ形 is undergoing an 音便 (sound-change). Does that help?
– ハイドン Apr 25 '14 at 02:39