First blood is literally the first shedding of blood especially in a boxing match or formerly in duelling with swords.
RELATED IDIOMS :
first among equals
the person or thing having the highest status in a group
This expression is a translation of the Latin phrase primus inter pares, which is also used in English.
first come first served
used to indicate that people will be dealt with strictly in the order in which they arrive or apply
as a first point
first of all – informal chiefly North American
1991 - Globe & Mail (Toronto) - First off, I wouldn't worry about the fashionability of any particular garment. If you'd like to wear something, then wear it.
first past the post
(of a contestant, especially a horse, in a race) winning a race by being the first to reach the finishing Line
denoting an electoral system whereby a candidate or party is selected by achievement of a simple majority – British
early in the morning
before anything else
first things first
important matters should be attended to before anything else
First Things First was the title of a book by George Jackson, subtitled'Addresses to young men (1894).
first of all
at the first attempt – Australian
get to first base
achieve the first step towards your objective – informal - chiefly North American
1962 - P. G. Wodehouse - Service with a Smile – She gives you the feeling that you'll never get to first base with her.
of the first order = of the first magnitude
used to denote something that is excellent or considerable of its kind
In astronomy, magnitude is a measure of the degree of brightness of a star. Stars of the first magnitude are the most brilliant.
of the first water
extreme or unsurpassed of kind
The clarity of diamonds is assessed by their translucence; the more like water, the higher the quality. The sense of water referred to in this expression is the quality of brilliance and transparency of a diamond or other gem. If a diamond or pearl is of the first water it possesses the greatest possible degree of brilliance and transparency. In its transferred use, however, the phrase often refers to someone or something regarded as undesirable, e.g. a bore of the first water.