A novel way to signal strong tea, its being so solid that it can support a mouse’s weight. The more common version of the same idea has the tea ‘so strong you could stand a spoon up in it’.
For another tea-related simile, less innocuous, see Mervyn Peake’s Titus Groan.
‘Tea… was so strong you could trot a mouse on it.’
Source: Laurie Lee, As I Walked Out One Midsummer Morning, Penguin 1971, p.29
Photo credit: IsraelBest at pixabay