Bouvier's Law Dictionary, Revised 6th Ed (1856):

SURPRISE. This term is frequently used in courts of equity and by writers on
equity jurisprudence. It signifies the act by which a party who is entering
into a contract is taken unawares, by which sudden confusion or perplexity
is created, which renders it proper that a court of equity should relieve
the party so surprised. 2 Bro. Ch. R. 150; 1 Story, Eq. Jur. Sec. 120, note.
Mr. Jeremy, Eq. Jur. 366, seems to think that the word surprise is a
technical expression, and nearly synonymous. with fraud. Page 383, note. It
is sometimes, used in this sense when it is deemed presumptive of, or
approaching to fraud. 1 Fonb. Eq. 123 3 Chan. Cas. 56, 74, 103, 114. Vide 6
Ves. R. 327, 338; 2 Bro. Ch. R. 826; 16 Ves. R. 81, 86, 87; 1 Cox, R. 340; 2
Harr. Dig. 92.
     2. In practice, by surprise is understood that situation in which a
party is placed, without any default of his own, which will be, injurious to
his interest. 8 N. AS. 407. The courts always do everything in their power
to relieve a party from the effects of a surprise, when he has been diligent
in endeavouring to avoid it. 1 Clarke's R. 162; 3 Bouv. Inst. n. 3285.