The phrase a snake in the grass denotes a treacherous person or harmful thing that is hidden or seemingly harmless.
It may ultimately be after the following from Eclogues, by the Roman poet Virgil (Publius Vergilius Maro – 70-19 BC):
Qui legitis flores et humi nascentia fraga,
frigidus, O pueri, fugite hinc, latet anguis in herba.
(translation: A. S. Kline – 2001)
You boys that pick flowers, and strawberries, near the ground,
run away from here, a cold snake hides in the grass.
The English phrase first appeared as the title of a book, published in London in 1696, by Charles Leslie (1650-1722), a nonjuring Church of Ireland cleric (i.e. a cleric who refused to take the oath of allegiance to William III and Mary II in 1689):
The snake in the grass: or, Satan transform’d into an angel of light. Discovering the deep and unsuspected subtilty which is couched under the pretended simplicity of many of the principal leaders of those people call’d Quakers
Thomas Smith (1638-1710), a nonjuring cleric and expelled Fellow of Magdalen College, Oxford, used the phrase in a letter dated 5th March 1709:
(from Remarks and Collections of Thomas Hearne – Oxford, 1886)
They are now striking at the foundations of the Colleges of both Universityes, under the pretense of having the Statutes repealed, wᶜʰ oblige the Fellowes to take H. orders: but it is visible, that there is a Snake in the grasse, and the designe is mischievous, upon the Supposition of their being established in the times of ignorance and Superstition: wᶜʰ will equally hold to diminish the number of Dignityes in Cathedrals, and by degrees draw on the sacrilegious invasion of their revenues, to maintaine this holy warre against Popery, and introduce Presbyterian parity & poverty among our Clergy.
The same idea was expressed by the obsolete phrase a pad in the straw, where pad, of Germanic origin, means toad. It is first recorded in the textbook Lesclarcissement de la langue francoyse (1530), by the teacher and scholar of languages John Palsgrave (died 1554):
There is a padde in the strawe. Il y a de loignon. Though they make never so fayre a face, yet there is a padde in the strawe: tant tiennent ilz bonne myne, or tant facent ilz bonne mine, si il y a de loignon.
The obsolete French phrase il y a de l’oignon, literally there is some onion, means there are obscure motives, suspicious events, which give an inkling that difficulties are afoot. Randle Cotgrave translated it, under the headword oignon, in A Dictionarie of the French and English Tongues (1611):
Il y a de l’oignon. There is a pad in the straw, there’s somewhat amisse among them.
The origin of this French phrase is obscure; perhaps the image is that such motives or events, like an onion, give one reasons to cry…