I've never heard this word before, and I can't figure out what it means, either. Perhaps I'm not picking up the clues, but I'm visualizing a yawn mixed with a chomp, but I truly doubt that's the meaning. Could you please enlighten me?

My Chambers defines it this way: "yomp, (esp. mil. coll.) v.i. to carry heavy equipment on foot over difficult terrain. [Poss. imit.]"

A quick search at Google reveals that the verb is now used very widely in British English to refer to hiking and tramping generally.