You would usually use it as a verb, "to grimace", as in... "The man grimaced when he realised how much he would have to pay". You can also use it as a noun it's just a little less common. He made a grimace is perfectly sound grammatically, however. "To mug" is similar to the verb "to rob". it's totally different. It usually refers to street muggings, (I'm referring to a robbery on the street, perhaps with a knife on the sidewalk...). I believe you are referring to the word "SMUG" which is different from the word "grimace". "Smugness" is a quality that one displays when they are "happy with themselves". It tends to show a lack of modesty ... it's a type of "showing off". "When Hugh showed up in his brand new Ferrari, you could see his smugness oozing through his smile and walk." = "When Hugh showed up in his new Ferrari, you could see how "happy he was with himself" through the way he smiled and walked." Hugh was probably not being very modest and was "showing off" his new Ferrari. Hope this helps, Take care and good luck!