One of the interpretations on Lane's Lexicon of the word 'Janabah' is:
He was, or became, in the state of one who is termed جُنُب; (S, IAth, Mgh, L, Msb, K;) i. e., under the obligation of performing a total ablution, by reason of sexual intercourse and discharge of the semen. (IAth, TA.)" [Edward Lanes].
However, I believe this came from hadith and inserted itself into the lexicons. This happens with Ghusl as well as Lane's lexicon made a statement that Ghusl means washing every part 3 times (which is sourced from a hadith).
With that said, I reject this non-quranic view of the word 'Janabah'. The quran explicitly clarifies what it means by Janabah...Twice.
[Qural 4:43] O you who have believed, do not approach prayer while you are intoxicated until you know what you are saying or in a state of janabah, except those passing through [a place of prayer], until you have washed. And if you are ill or on a journey or one of you comes from the place of relieving himself or you have contacted women and find no water, then seek clean earth and wipe over your faces and your hands [with it]. Indeed, Allah is ever Pardoning and Forgiving.
On the underlined quote above, it is clear the Quran says [paraphrasing] "if you happened to be dirty and not find water, do this alternative thing". So the Quran explained what it means to be dirty (Janabah). You are in a state of Janabah: when you are ill (sickness and infectious processes), Traveling (imagine a 7th century Arab traveling for days--very dirty), come from a bathroom (imagine 7th-century toilets and their dirtiness), or had contacted women. These are what constitute a state of Janabah. And if you don't have water when you are in that state, then use some clean earth and wipe face and hands.
[Quran 5:6] O you who have believed, when you rise to [perform] prayer, wash your faces and your forearms to the elbows and wipe over your heads and wash your feet to the ankles. And if you are in a state of janabah, then purify yourselves. But if you are ill or on a journey or one of you comes from the place of relieving himself or you have contacted women and do not find water, then seek clean earth and wipe over your faces and hands with it. Allah does not intend to make difficulty for you, but He intends to purify you and complete His favor upon you that you may be grateful.
Again it clarified what that word Janabah means. It elabored on what to do if you don't find water and can't wash yourself when in Janabah.
In conclusion: I believe the Lexicon's inclusion of Ghusl to mean washing every body part 3 times, and Janabah to mean after having sexual intercourse/seminal discharge is based on hadith influence. It also means being in a journey, and ____, ____, etc. The Quran clarified this above right after discussing what to do when in Janabah.
I prefer JosephIslam's input on this. Anyone else, please comment on this and what you feel.
I believe this is very simple. "If you are dirty: wash yourself before prayer. And if you cannot find water do x,y,z".