1590s, "not firm or secure in position," from un- (1) "not" + steady (adj.). Similar formation in Old Frisian unstadich, German unstätig, Middle Dutch onstadich. Meaning "marked by irregularity" is from 1680s. Related: Unsteadily (1550s).
"make unsteady," 1530s, from un- (2) "reverse, opposite of" + steady (v.). Related: Unsteadied; unsteadying.
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