quickly: at high speed or after only a little time; speedily, soon, in haste
This is the plain adverb "quickly," and it carries two shades that English usually splits into different words. It can describe the speed of an action, as when Rahab tells the king's men to "pursue quickly" (Joshua 2:5), and it can describe how little time passes before something happens, as in the repeated Deuteronomy threat that a disobedient Israel will "soon" perish from the land. Both shades come from one idea: little time between the word and the deed.
A striking cluster of its uses is the fixed phrase "they turned aside quickly from the way" (Exodus 32:8; Deuteronomy 9:12, 9:16; Judges 2:17). There the force is less about physical speed than about shock: Israel abandoned the covenant so soon after receiving it. When the word points at people acting rashly, that negative tint (moving too fast, before thinking) comes from the setting, not from a second meaning of the word itself, as in the warning not to rush into a lawsuit (Proverbs 25:8).
The word is the adverb built from the verb מָהַר, "to hurry." It should not be confused with the look-alike adjective מָהִיר, "swift" or "skilled," which describes a capable person rather than the pace of an action.