1.莽撞冲动地行事, Be irrationally carried away, act irresponsibly or heedlessly
2.走向极端，歇斯底里go to extremes, go too far, lose one's good sense
For example, Just because you like her looks doesn't mean you should go off the deep end and propose.
大家知道游泳池一端是深水，另一端是浅水。如果这人又不识水性，跳进了没顶的深水中，那么他想必是相当莽撞冲动的人。让我们听个例子来体会go off the deep end这个习惯用语用来比喻什么样的行为。这是爸爸在为热中于一种跳伞运动的儿子Bob担心。作这种跳伞运动的人在空中尽量延缓张开降落伞的时间。例句Bob has gone off the deep end about skydiving. We tell him it's a dangerous sport but he goes up and jumps every weekend. We're really worried sick until we see him safely back home. 他说：Bob去作那种跳伞运动可真是莽撞冲动。我们跟他说这种运动很危险，但是他还是每个周末登高去跳。我们担心得要命，直到见他回家才把悬著的心放下。
