Diminutive of Jane, from Hebrew Yochanan meaning God is gracious.
Janey is the sunniest, most informal member of a large and ancient family. Its journey begins with the Hebrew 'Yohanan' — 'God is gracious' — which passed through Greek as 'Ioannes,' through Latin as 'Joannes,' into Old French as 'Jehanne,' and eventually crystallized in English as both 'John' (the masculine form) and 'Jane' (the feminine). Jane itself was the dominant feminine form for much of the English-speaking world by the Tudor period, borne famously by three of Henry VIII's wives in various forms, by Jane Austen, and by the archetypal everywomen of literary tradition.
Janey is simply Jane with a warm suffix attached — intimate, affectionate, slightly breezy. The '-ey' ending signals familiarity in the English naming tradition: it's what you call someone you like. Janey appears throughout nineteenth and early twentieth-century fiction as the name given to plucky, approachable heroines — women with more personality than pretension.
It has an almost musical quality compared to the clipped monosyllable of Jane, yet none of the added formality of Janet or Janette. In contemporary use, Janey occupies an unusual space: it is rare enough to feel genuinely individual but immediately legible to any English speaker. It suits an era that values warmth and unpretentiousness over elaborate invention. Parents who find 'Jane' too spare and 'Janet' too mid-century are rediscovering Janey as a registration name that carries its nickname built in — cheerful, rooted, and impossible to mispronounce.