With all one's strength, ability, or resources; not holding back. For example, They are going all out to make the fund-raiser a success. This seemingly modern term dates from about 1300, when it meant "completely" or "wholly." It now refers to making a great effort and is also used adjectivally, as in an all-out effort. This usage became current in America in the late 1800s, with reference to races and other kinds of athletic exertion. In the mid-1900s it gave rise to the phrase to go all out and was transferred to just about any energetic undertaking. Also see go whole hog.
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