From the gardeners at the South London Botanical Institute,  24 June  2012:

At a garden opening yesterday a visitor asked where the name toadflax comes from. The best anyone could think of is that its flower, when squeezed, might be considered to resemble a toad’s mouth; thus it is similar to the names bunny-mouth, dog’s mouth, gap-mouth, lion’s mouth and mice’s mouth, etc., which have been given to common toadflax (Linaria vulgaris) – see the Local Names page on this website.
Can anyone suggest any alternative explanations?

Responses
1. According to James Britten & Robert Holland in their Dictionary of English Plant-names, 1886: William Coles in his Adam in Eden, 1657: 313, ‘hazards the conjecture that it was given “because toads will sometimes shelter themselves amongst the branches of it”.’ [RV, June 2012].

2. According to John E. Stevens in his Discovering Wild Plant Names, 2008: 56:
‘The leaves have been likened to those of flax [Linum], which accounts for part of the common name, but why the prefix ‘toad’ has been applied is uncertain. It seems probable that it arose from an early translation error – Dodoens described the plant as ‘Herba asimilis cum bubonio facultatis‘, meaning that the plant was used to treat hot swellings called ‘buboes’ and was therefore given the name bubonium. In translation this name could have been mistaken for bufonium which derives from the Latin bufo, ‘toad’. Other suggestions for the name are that it simply means spurious flax, or that toads like to shelter beneath its leaves’ [Josie Watson, July 2012].

3.  According to Ron Freethy, From Agar to Zenry, 1985:  ‘Toadflax was once a destructive weed of flax fields.  Before it flowers this attractive-looking plant is difficult to separate from the crop, by which time its strong creeping rootstock makes it difficult to get rid of.  Hence the insulting name given to it’ [RV, September 2016].

4.  According to Mario Maculan, in the London Natural History Society Newsletter, 267: 21, November 2022: ‘The “flax” part of the English name reflects the shape of the leaves, while the “toad” part implies uselessness’ [RV, December 2022].

5.  Gareth H. Browning, in his Book of Wild-flowers and the Story of their Names, 1936, explains:  ‘You can easily discover a connection with the toad.  If you press open the lip of the flower, for then they look very much like the wide mouth of that reptile [sic.].  Even if you look at the flower from underneath you will simply be struck by its suggestion of the mouth of a toad or frog’ [RV, January 2023].

Image:  common toadflax, Linaria vulgaris, The Black Country Living Museum, Dudley, West Midlands, September 2015.

Updated 24 January 2023.