1
general::
Reduce resistance, as in His sales motto was: a fine lunch and a few drinks often will soften up a prospective customer. This expression transfers lessening of physical hardness to lessening mental resistance. It was first used, however, in World War II, where it meant "reduce the enemy's defenses by preliminary bombing." [c. 1940]
American Heritage Idioms
2
general::
Phrase(s): soften up
1. Lit. [for something] to become softer. • The butter softened up in the heat of the day. • The candles will probably soften up and bend over in this hot weather. 2. Fig. [for someone] to adopt a more gentle manner. • After a while, she softened up and was more friendly. • It was weeks before Ted softened up and treated us more kindly., Phrase(s): soften something up
to take actions that will make something softer. • Soften the butter up before you add it to the batter. • Please soften up the ice cream before you try to serve it., Phrase(s): soften someone up
Fig. to prepare to persuade someone of something. • I will talk to Fred and soften him up for your request. • I will soften up your father before you ask him about it.
McGrawhill's American Idioms And Phrasal Verbs