After working too long under the hot sun, he had a sudden blackout and nearly fell over.

blackout 是名词，由形容词 black（黑色的）和动词 out（熄灭）组成，字面意思是"变黑、失去知觉"。在这里指由于过度劳累或高温导致的"短暂意识丧失"或"眩晕"。医学上也称为"晕厥"。