信子apparently can mean (snake's)"tongue" but only if 吐 also occurs near it (s. all examples below) http://news.sina.com.cn/s/p/2014-08-03/051130621113.shtml headline：半夜如厕 小臂粗大蛇 蜷缩地上吐信子 within text：（１）一条两米长的蛇盘在空调上，嘴里“嘶嘶”吐着红信子…… （２）不停游动，还吐着红信子 （３）发现一条两米左右的大蛇正对着自己吐信子。 iciba has 网络 nobuko (日本女孩的名字)；Hyacinth and １ example 它正在用它的巨大的、恐怖的眼睛凶猛地环望四周，鲜红的信子一吞一吐。1. It looked around savagely with its gigantic, mesmerizing eyes bright red tongue hung out, twitching. Searching internet for 信子 results in first 100 results all about 风信子 (hyacinth)
I believe 信子 is a colloquial and maybe dialectal word. It appeared in an online news headline doesn't guarantee its formality, since language usage for online news in China is far less regulated than TV news or newspaper. As a matter of fact, I've seen many regional words and network meme being used on Sina news in the past.
For the etymology, I couldn't find any authoritative reference, but I think the word was deviated from 芯子 which has a similar pronunciation, literally means 'the inside', and is usually used to refer to the wick - the mandrel of any cylinder-shaped object. Looking at the examples from ZDic, 信子 is definitely not exclusively used for snake tongues, but is almost interchangeable with 芯子:
Maybe you are looking for an explanation why "信子" can mean tongue of a snake in some special context?
If so, actually, "信子" of a snake has nothing to do with “风信子”(hyacinth). It is just a conventional phrase when talking about tongue of snake, and this convention is rarely used now.
Why tongue of a snake is called "信子"? A possible explanation is that tongue is an important thing for snake to detect surrounding circumstance the as most snakes are weak-sighted, and "信" represents "information" in Chinese, "子" is "an item used to do something", like "锤子"(hammer), "铲子"(spade). Therefore "信子" might mean "a tool for detecting information" for snakes.
And similarly pronounced 芯子
搜狗 explains it like this:
Which basically says that snakes obtain information about the outside world through their tongues so it's their "little messenger," poetic license and all.