Wiktionary
vb. 1 To make#Verb softer. 2 (context by extension idiomatic English) To appease someone in order to make them more receptive to an idea or proposal.
Usage examples of "soften up".
He used his dementia monsters to soften up and distract an opponent, then moved into melee for the win.
Having this grotesque locale disgusting with rotting corpses and such is a good way to soften up a victim's resolve.
Already the Quman archers fire at will, to soften up their enemy, but the Ungrians give as good as they get, and the Wendish legions swing wide and begin a steady advance toward the flanks.
Even the claws on his alligator foot, which had grown to respectable size now, were starting to soften up.
When the ship uses the air for braking, or when the sun shines on it in space, we'll need something that won't soften up at ordinary temperatures.