防不胜防（防不勝防）　

◎ 防不胜防 fángbùshèngfáng

[impossible to defend effectively;cannot reckon with all eventualities;be not easy to guard against;cannot stand up to] 要防的事太多,提防不过来

这种小人,真是防不胜防。——清· 吴趼人《二十年目睹之怪现状》