Excessively; extra; outer; above.
The form has several of the senses of the preposition over and appears in a very large number of compounds that can be nouns, adjectives, verbs, or adverbs. Over- can be freely prefixed to other words for a momentary need. Its meanings are rather variable and diffuse, and difficult to categorize.
Having said that, there are some groupings that are commonly encountered. The one most often found refers to something beyond what is usual or desirable, even excessively so (overambitious, overcareful, overexert, overindulgence, overfull, overprecise, overprepared, overweight), which leads into a rarer sense of ‘utterly’ or ‘completely’ (overawed, overjoyed). In others, the form has a spatial sense of something above or higher up, which broadens into a figurative sense of something that is superior (overhang, overbridge, overarching, overlook, overtone, overlord, overrule). A fourth set is of words in which the meaning is of something upper, outer, or extra (overcoat, overshoes, overtime). Another sense is of motion forward and down, and hence of inversion (overturning, overbalance, overthrow, overboard). The form can also suggest covering a surface (overpaint, overgrow).