Word/phrase for talking tough when afraid

I haven't found a general-purpose term for this. However, it is somewhat of a storytelling trope.

TV Tropes calls this trope Twice Shy, with the connotation that the two people are either too shy, too awkward, or too afraid of the circumstances if their love were to be revealed.

They also list a variation, Belligerent Sexual Tension, in which each person refuses to admit their love (sometimes not even to themselves) and takes the energy out as aggression toward the other instead.

Habituated, past participle of habituate (“To turn into a habit, to make habitual”), is sometimes used. See, for example, the Habituation wikipedia article for examples of habituation in animals, such as “the response becomes habituated if the stimulus repeatedly occurs but causes no harm. An example of this is the prairie dog habituating to humans”.

Animalbehavioronline provides a similar example, remarking that prairie dogs “collectively ... are quite well-defended, as their alarm calls facilitate escape in burrows. When prairie dog towns are located near trails used by humans, giving alarm calls every time a person walks by is a waste of time and energy for the group. Habituation to humans is an important adaptation in this context.”