Goku said that Nappa was a tough bastard because he believed Nappa should have taken some damage from their brief beam struggle. The definition of tough is as follows:
tough/təf/
adjective
1. (of a substance or object) strong enough to withstand adverse conditions or rough or careless handling.
Do not try and alter the definition of the word to suit your bias, it in no way reflects strength, Ki or battle power. It refers to what I explained above and nothing more.
I just went through the Strength Checker: http://www.kanzenshuu.com/forum/viewtopic.php?f=8&t=15488 and it's Goku who uses the terminology tough, not Vegeta.
Goku: “He really is a tough bastard! Even though he should have been hit a little bit by my Kamehameha!! At this rate this looks like it will take forever...”
Because Nappa wasn't phased from their brief beam struggle, due to being tough; durable, Goku states '
at this rate', which places special emphasis on the fact that Nappa wasn't hurt from their previous encounter and it was because of that, that he states that it '
looks like' it will take forever.
However, Vegeta immediately calls Nappa down directly after he uses his Ultimate attack. The suggested implication here is that Vegeta was banking on that, as was Nappa, but Goku countered it and falsely believed that it was possible that Nappa had more to give, Vegeta knew other-wise.