What type of training was Class E's Yuuma going to say, before he corrected himself? Assassination Classroom - Chapter 24, page 18, second panel

What was the character about to say?
In Japanese, the panel reads:

ありがとねぇ　ジャマな枝　切ってくれて
それにしても　君達えらく　身軽だね～
いえいえ
待ち伏せ…いや　木登り特訓　してるので

The translation in the OP post is a little bit off; a better translation of the last speech bubble is &quot;It's because we were preparing an ambu-... er, practicing climbing trees.&quot; That is, they weren't doing &quot;ambush training&quot;; they were just lying in wait to ambush someone. (In other words, it's not 待ち伏せの特訓 that was elided - just 待ち伏せ.)
Anyway, the word in question is ambush.