| усмехаться/усмехнуться в усыa | to hide a grin behind one's moustache, to grin into one's moustache |
| улыбаться/улыбнуться в усыa | to smile into one's moustache |
NB The above expression indicates that the subject is smiling to himself. It is used only when the person really has a moustache. | |