But it quite literally is:

    insecure (adj.)
    (of a thing) not firm or fixed; liable to give way or break.
    not sufficiently protected; easily broken into.

A webpage loaded over HTTP is easy to tamper with. Let me give you an example of traffic over HTTP that is secure -- apt repositories; because you're only retrieving payloads protected by PGP, so the actual payload is firm, fixed, and not easily broken into.

How else do you define insecure? Have I misunderstood the definition?