| disreputable(shaky, unlikely, unpromising) |
| dynamite(adjective, delicate, uncertain, unstable, doubtful) |
| endangered(delicate, uncertain, sensitive, unsound, tricky) |
| enigmatic(uncertain, doubtful, dubious, tricky, sticky) |
| error-prone(adjective, unreliable, insecure, treacherous, incalculable) |
| experimental |
| fair-weather(adjective, unreliable, insecure, treacherous, incalculable) |
| fairweather(adjective, unreliable, insecure, treacherous, incalculable) |
| faithless(adjective, unreliable, insecure, treacherous, incalculable) |
| fallible(adjective, unreliable, insecure, treacherous, incalculable) |
| far-fetched(uncertain, doubtful, shaky, unsure, unlikely) |
| fast and loose(adjective, unreliable, insecure, treacherous, incalculable) |
| fat chance(expression, shaky, unlikely, unpromising) |
| fatal |
| fatidic |
| fitful(adjective, uncertain, unstable, irregular, doubtful) |
| flash |
| fly by night(adjective, unreliable, insecure, treacherous, incalculable) |
| fortuitous |
| fraught with danger(expression, adjective, idiom, delicate, uncertain) |