Being oblivious to something that should otherwise be easily seen.
This phrase is said to date back to at least the 1500s, but the earliest I could find it is only as far back as the 1600s. The term is found in The Copie of a Letter sent from The Roaring Boyes in Elizium, from the year 1641:
"Can you my worthy hosts sit and see those
That make you weare od money in your hose,
Under your nose triumphing?"
A common alternative to this phrase is 'right under your nose, as seen in the The Sydney Morning Herald, November 1856:
"'Well, have you detected my forgery?' said he, with the greatest coolness, 'No,' replied the head
expert laughing, 'for a good reason—you never sent it.' 'Why, there it lies right under your nose,
the third to the left, and here is the original I took it from.'"
* Heather lost her car keys and was searching all over her house trying to find them. However, she did not realize that her keys were on top of her desk in plain sight the entire time, right under her nose.
* Even though the pizza cutter was right under Jake's nose, he couldn't find it, so he used a sharp knife to slice the pizza instead.
Note: The origins for some phrases are not clear. In such cases, I try to provide the plausible theories that take a guess as to how a phrase may have originated, but don't take it as a full confirmation. Sometimes, I may not even comment about a phrase's origins, other than just showing a quote which contains the oldest written form of the saying that I could find. This is to give you an idea on how old some of these expressions are, however, keep in mind that an older recording may be gernally known and I just happened to miss it.