1590s, "to start with fright (as a startled horse does), shy, take alarm," from Middle English bugge "specter" (among other things, supposed to scare horses at night); see bug, n.; also compare bogey, n.1. The meaning "to raise scruples, hesitate" is from 1630s. As a noun from 1650s. Related: Boggled; boggling; boggler (from c.1600 as "one who hesitates").