To separate the connected parts of.(verb-transitive)
To become loosened or separated.(verb-intransitive)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "unfasten" in a sentence
"One of the Indians stepped up and proceeded to unfasten his pack-straps."
"He let her go long enough to unfasten his necklace."
"All those little frog fastenings—he itched to unfasten them one by one."