前缀over-的前身是英语介副词over, 它的基本意思是 above (在…之上），由此又引申出across (越过），too (太），too much (过分）等意思。over-作为修饰性的派生前缀，能与形容词 (或分词）、名词及动词缀合，一般不改变词基单词的词性。over- 与形容词（或分词）结合时，意思大致为too;与名词结合时，意思大致为over, above (这时词基的词性可能发生变化）或too much, excessive;与动词结合时，意思大致为over或too much。

先介绍几个over-与形容词（或分词）结合的词例。

overactive [over- = too 太; active a. 活跃的→]a. too active; active to excess 过于活跃的 / Most of the children there were overactive, overirritable and doing poorly scholastically. 那儿的大多数儿童都多动，过于易怒，学业都很差。

overdue [over- = too 过分；due a. 到期的=>“too due 过分到期了”→] a. more than due; due some time ago but not yet paid 过期的；到期未付的/This bill is overdue.这张支票过期了。

overbearing [ over- = too 过分；bearing a. 压制的=>“too bearing 过于压制人的”→]a. forcing others to one's own will；masterful 好压制的；专横的 / We found it hard to like the new teacher because of his overbearing manners. 我们觉得很难喜欢上新来的老师，因为他作风专横傲慢。

接着介绍over-与名词缀合的（改变词性与不改变词性）两种情况。

overboard [ over- = across 越过；board n. 船舷=>“across the board 越过船舷的”→]ad. from ship or boat into water 从船上落人水中 / The sailor slipped and fell overboard. 那船员滑了一跤落人水中。 

overhead [ over- =above 在…上；head n. 头→]ad. over the head; far above 在头顶上；架空的 / You can see a cableway overhead. 你能看到头顶上有条高架索道。