The Collaborative International Dictionary
handfast \hand"fast`\, v. t. [imp. & p. p. handfasted; p. pr. & vb. n. handfasting.]
To pledge; to bind. [Obs.]
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To betroth by joining hands, in order to permit cohabitation, before the formal celebration of marriage; in some parts of Scotland it was in effect to marry provisionally, permitting cohabitation for a year, after which the marriage could be formalized or dissolved.
Note: Handfasting was a simple contract of agreement under which cohabitation was permitted for a year, at the end of which time the contract could be either dissolved or made permanent by a formal marriage. Such marriages, at first probably not intended to be temporary, are supposed to have originated in Scotland from a scarcity of clergy, and to have existed at times in other countries.
Wiktionary
vb. (en-past of: handfast)