E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Several flowers are so called. Red Bachelors Buttons, the double red campion; yellow Bachelors Buttons, the upright crowfoot; white Bachelors Buttons, the white ranunculus and white campion.
The similitude these flowers have to the jaggod cloath buttons anciently worne gave occasion to call them Bachelours Buttons.Gerard: Herbal.
Or else from a custom still sometimes observed by rustics of carrying the flower in their pockets to know how they stand with their sweethearts. If the flower dies, it is a bad omen; but if it does not fade, they may hope for the best.
To wear bachelors buttons. To remain a bachelor. (See above.)