Think of a hard word with its sharp edge filed off. Son of a gun began as a polite swap for a much ruder phrase, and swapping one word took out the sting while leaving the force. So what you have is a big feeling with nothing dangerous in it — and a big feeling can point anywhere. Aim it at a friend across the room and it is a hug. Aim it at the man who kept you waiting and it is a complaint. Aim it at a jar lid that will not turn and it is a joke. Let it out with no target at all and it is just the noise surprise makes. The words never change; the aim and the voice do everything.
The source records son of a gun as a euphemistic variant of son of a bitch. It also notes nautical folk etymologies — a child born aboard between the guns, or fathered by a soldier — and says plainly that these are not supported by evidence.
來源記載 son of a gun 是 son of a bitch 的委婉替換版，這正好解釋了它為什麼火氣比較小。至於流傳很廣的水手說法——船上大砲之間出生的孩子，或士兵生的孩子——來源明說沒有證據支持，所以當故事聽就好，不要當來歷教。
Wiktionary entry for “son of a gun”, etymology_text: “Euphemistic variant of son of a bitch… these etymologies are not supported by evidence.” (packet source_ref kaikki:3f6f69a190036ae4:1178873287:4411)
Three branches
丟向一個男人
最原始的方向：這句話本來就是用來講人的。同樣的字往人身上丟，可以是熱情，也可以是不爽，中間沒有第三種字可選，只能靠語氣分。
noun · B2
A joking, friendly name for a man you like or admire. It sounds rough, but the feeling behind it is warm — you say it with a smile, often to an old friend or to someone who has just done something impressive.
好傢伙
丟向一件東西
把本來給人的稱呼改丟給物品：那個罐蓋好像故意跟你作對。東西不會真的故意，所以這一丟聽起來是玩笑，不是動怒。
noun · C1
A thing that is giving you trouble and will not do what you want. It is said with a laugh rather than real anger, as if the object were a stubborn person.
難搞的東西
沒有對象，直接喊出來
把對象整個拿掉，剩下的就是那股情緒本身。所以它變成一聲驚呼，跟人和東西都無關。
interjection · B2
Said out loud when something catches you by surprise. Most often it is happy or startled surprise, but people also say it when something has just gone wrong, or when they are annoyed or disappointed.
哇／真是的