The antonyms for the word "soften up" are those words that signify the opposite of making something gentle, tender or sympathetic. Such words include harden, toughen, firm up, vitalize, invigorate, stimulate, and embolden among others. To harden up is to make something stronger, more resistant or unyielding. Toughen up implies the strengthening of something which was initially weak or fragile. Firm up on the other hand means to stabilize and make something more solid or inflexible. Vitalize and invigorate suggest the renewal of something's energy or strength. While stimulate and embolden mean to create a sense of excitement or courage in someone causing them to be more active or adventurous.