It comes from Shakespeare:
Of moving accidents by flood and field.
- Othello the Moor of Venice (Othello at I, iii)
From the text, I think it means 'accidents that occured in fields or with floods'. :D
Her father loved me; oft invited me;
Still question’d me the story of my life,
From year to year, the battles, sieges, fortunes,
That I have passed.
I ran it through, even from my boyish days,
To the very moment that he bade me tell it:
Wherein I spake of most disastrous chances,
Of moving accidents by flood and field,
Of hair-breadth ’scapes i’ the imminent deadly breach,
Of being taken by the insolent foe
And sold to slavery, of my redemption thence
And portance in my travels’ history;
Wherein of antres vast and deserts idle,
Rough quarries, rocks and hills whose heads touch heaven,
It was my hint to speak,—such was the process;
And of the Cannibals that each other eat,
The Anthropophagi, and men whose heads
Do grow beneath their shoulders. This to hear 2
Would Desdemona seriously incline.