Despite their surface similarities, diffuse and defuse have very different origins.
With defuse, that very helpful prefix de has been added to fuse. De came to English via Latin and French, and one of its most common uses is to indicate an opposite action. Defuse, according to Etymology Online, became widely recognised around 1943. World War II must have presented rich opportunities for defusing explosive situations.
1. remove a fuse from an explosive device
2.reduce tension or potential danger
A web definition of ‘defuse’
With Brackett’s escape foiled, he had little choice but to help Jim defuse the bomb that he’d set up for his extortion attempt.
Blair started talking a mile a minute, hoping to distract Jim and defuse some of the tension building in his friend.
Diffuse is a word that comes to us from the sixteenth and seventeenth centuries, via the Latin diffusus, meaning pouring out or away, and is related to words such as diffusion.
As an adjective
1. Spread out, not concentrated (as of light, inflammation etc)
2. not concise, verbose, long-winded
As a verb
1.to disperse or be dispersed from a centre
2. spread or be spread widely, to reach a large area
A web definition of ‘diffuse’
Whatever drug Lash had forced down Blair’s throat acted quickly; a terrifying weakness began to diffuse through his body.
Simon had noted that Blair had a diffuse way of getting to the point of a conversation, and today he had no patience for it. “The short version, Sandburg,” he commanded.
Diffuse is the longer word by one letter – so if you’re writing with two ‘f’s, ask yourself if your intended meaning is something spread out or dispersed. Alternatively, remember that the prefix de indicates an opposite or negative action. If you want the opposite of things going boom, whether the explosion is chemical or emotional, you want defuse.
Definitions are courtesy of my New Zealand Oxford Dictionary.