birth of a neologism

I've never read Dylan Thomas' Under Milkwood, but recently I came across an extract from it and fell in love with one word - sidespies. Where has this expression been all my life? (Hiding in an unread book, obviously.)

Since I saw the word, I often want to use it, because it so exactly expresses a certain look that side glance doesn't even come close to defining.

But is it mine to use? If I were to use it in my own writing, would I be a plagiarist? When does a word become common property? When is a neologism available for use? Macquarie Online Dictionary defines a neologism:

noun 1. a new word or phrase:

2. the introduction or use of new words, or new senses of words.

By the way, Macquarie doesn't have sidespy, and neither did any online dictionary I looked at. The only place I came across an online reference to the word is on a thread at Wordreference.com.

We all use many words coined by Shakespeare. I think it's time to get Thomas' great little verb out into the general world.

Here's the extract I read:

        FIRST VOICE

In the blind-drawn dark dining-room of School House, dusty
and echoing as a dining-room in a vault, Mr and Mrs Pugh
are silent over cold grey cottage pie. Mr Pugh reads, as
he forks the shroud meat in, from Lives of the Great
Poisoners. He has bound a plain brown-paper cover round
the book. Slyly, between slow mouthfuls, he sidespies up
at Mrs Pugh, poisons her with his eye, then goes on
reading. He underlines certain passages and smiles in
secret.

        MRS PUGH

Persons with manners do not read at table,

        FIRST VOICE

says Mrs Pugh. She swallows a digestive tablet as big as a
horse-pill, washing it down with clouded peasoup water.

                                                     [Pause

        MRS PUGH

Some persons were brought up in pigsties.

        MR PUGH

Pigs don't read at table, dear.

        FIRST VOICE

Bitterly she flicks dust from the broken cruet. It settles
on the pie in a thin gnat-rain.

        MR PUGH

Pigs can't read, my dear.

        MRS PUGH

I know one who can.

        FIRST VOICE

Alone in the hissing laboratory of his wishes, Mr Pugh
minces among bad vats and jeroboams, tiptoes through
spinneys of murdering herbs, agony dancing in his
crucibles, and mixes especially for Mrs Pugh a venomous
porridge unknown to toxicologists which will scald and
viper through her until her ears fall off like figs, her
toes grow big and black as balloons, and steam comes
screaming out of her navel.

        MR PUGH

You know best, dear,

        FIRST VOICE

says Mr Pugh, and quick as a flash he ducks her in rat
soup.

        MRS PUGH

What's that book by your trough, Mr Pugh?

        MR PUGH

It's a theological work, my dear. Lives of the Great
Saints.

        FIRST VOICE

Mrs Pugh smiles. An icicle forms in the cold air of the
dining-vault.

Isn't it fantastic? I shiver just reading it.

If you want to read Under Milkwood, you can legally download it here for free.