1.不期而遇
I came across an old friend of mine in a d epartment store.
(chance to meet，bump into，come upon，meet...accidentally/unexpectedly/by chance/by accident)
2.不速之客
You are really a thick-skinned and gate-crasher.
3.不义之财
Don't be envious. Ill-gotten money comes easily and goes easily too.
4.不谋而合
see eye to eye：Do you see eye to eye with him on that plan?
5.不厌其烦
takes the trouble：The teacher often takes the trouble to instruct his pupils.
6.不遗余力
But the conspirators leave no avenue unexplored to re-establish contact with Britain.
7.不择手段
by hook or by crook：He just did the work by hook or by crook.
8.不翼而飞
Did you see that plane? It seems that it just vanished from sight.
9.不声不响
as quiet/silent/still as a mouse：She was as quiet/silent/still as a mouse and left.
10.不识抬举
Bring a cow to the hall and she will run to the byre. (fail to appreciate sb's kindness/not know how to appreciate favor)
注：
thick-skinned： 厚皮的，厚脸皮的，不敏感的