late Middle English: from Latin diffus- 'poured out', from the verb diffundere, from dis- 'away' + fundere 'pour'; the adjective via French diffus or Latin diffusus 'extensive', from diffundere
The verbs diffuse and defuse sound similar but have different meanings. Diffuse means, broadly, ‘disperse’, while the non-literal meaning of defuse is ‘reduce the danger or tension in’. Thus sentences such as Cooper successfully diffused the situation are regarded as incorrect, while Cooper successfully defused the situation would be correct. However, such uses of diffuse are widespread, and can make sense: the image in, for example, only peaceful dialogue between the two countries could diffuse tension is not of making a bomb safe but of reducing something dangerous to particles and dispersing them harmlessly.