The idiomatic English phrase “as one man” refers to several people doing something at once, and in harmony with one another. When an English speaker or writer refers to doing something “as one man”, the idea is that multiple people are acting simultaneously, or “all together.” A similar alternate meaning for this phrase is that a group of people agree on something totally, or feel the same about it and act decisively as a group.
Many times, English readers might use the phrase “as one man” to refer to physical activities that seem choreographed, or other cases where people act in a concerted way. A technical synonym for this is the phrase “in unison.” For example, someone who is watching a well choreographed dance routine, or similar performance, might say that the performers are acting “as one man.”
Another common use of the phrase is related to the decision making that happens in planning and administration. Regardless of the field or context, a group of people such as a board or committee might be said to be acting “as one man” if they produce a unilateral finding or a unanimous ruling on an issue. Another synonymous phrases for this includes, “of one mind,” which also indicates a group of people has the same thoughts and feelings on a matter and are in total agreement.
In these uses of the phrase “as one man,” another very similar traditional English and Irish phrase applies. The phrase “to a man” means that each person in a group is included. For example, if somebody says “they were all exhausted, to a man” it indicates that the condition of being extremely tired applies to each individual in the referenced group. Here, the two phrases can often coincide, where somebody might say that “they all agreed, to a man” and so “they acted as one man.”
In addition to these phrases, many others have been developed to illustrate a group of people acting together. Some are more technical and literal, such as “assonant” or “by acclamation,” while others are more figurative. For example, someone may talk about a group being “cheek by jowl” on a matter. All of these are fairly old and rather obsolete in many English speaking communities, where simpler phrases like “all together, “at once,” or “in unison” are much more popular.