You can think of squarely as the corner of a square: two lines meeting at a clean right angle, with nothing leaning and nothing sliding off to the side. Hold that picture and turn it towards a person, and you get someone facing you straight on. Turn it towards an object, and you get a thing set down dead centre. Turn it towards an idea, and you get blame or support landing on one place, all of it, with no room left to argue. Every use of the word keeps the same promise: no tilt, no edge, no half-way.
From the word’s history
Wiktionary records squarely as square + -ly, with the first use around 1557 in the writings of Robert Recorde. The right-angle picture inside square is therefore the oldest layer of the word, and the directness and completeness senses can be read as that picture applied to people and to ideas.
Wiktionary 記錄 squarely 由 square 加上 -ly 組成，最早的用例約在 1557 年，出現在 Robert Recorde 的著作裡。square 本身的直角畫面因此是這個字最早的一層；「正面面對」和「完完全全」可以看成同一個畫面被用到人和事情上。
Wiktionary entry 'squarely' (kaikki snapshot 3f6f69a190036ae4): 'From square + -ly. First use appears c. 1557 in the writings of Robert Recorde.'
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