The name Strength and Harmony in Chinese / Japanese...

Hapkido or 合氣道 is a mostly-defensive martial art in Korea.

Hapkido has some connection to the Aikido of Japan. They are written with the same characters in both languages. However, it should be noted that the Korean Hanja characters shown here are the traditional Chinese form - but in modern Japan, the middle character was slightly simplified.
Note: You can consider this to be the older Japanese written form of Aikido. Titles on older books and signs about Aikido use this form.

The connection between Japanese Aikido and Korean Hapkido is muddled in history. The issue is probably due to the difficult relationship between the two countries around WWII. Many Koreans became virtual slaves to the Japanese during that period. After WWII, many things in Korea were disassociated from having any Japanese origin. The relationship has greatly mellowed out now.

Looking at the characters, the first means “union” or “harmony.”
The second character means “universal energy” or “spirit.”
The third means “way” or “method.”
One way to translate this into English is the “Harmonizing Energy Method.” This makes sense, as Hapkido has more to do with redirecting energy than fighting strength against strength.

More Hapkido info

More notes:
1. Sometimes Hapkido is Romanized as “hap ki do,” “hapki-do” “hab gi do” or “hapgido.”

2. Korean Hanja characters are actually Chinese characters that usually hold the same meaning in both languages. There was a time when these characters were the standard and only written form of Korean. The development of modern Korean Hangul characters is a somewhat recent event in the greater scope of history. There was a time when Chinese characters were the written form of many languages in places known in modern times as North Korea, South Korea, Japan, Vietnam, Singapore, Hong Kong, Taiwan, Mainland China, and a significant portion of Malaysia. Even today, more people in the world can read Chinese characters than English.

3. While these Korean Hanja characters can be pronounced in Chinese, this word is not well-known in China and is not considered part of the Chinese lexicon.

力愛不二 is a proverb that literally means:
“Strength [and] Love [are] Not Two [separate ideas/concepts/things].”

You'll find this proverb translated from Japanese to English as:
Love and strength are not separate.
Power and love are indivisible.
Strength and love in harmony.
Strength and love stand together.

Old Japanese grammar is quite different than English, and so this proverb says a lot within the brevity of just 4 characters. If you just read these characters directly as “Strength Love Not Two,” you'd probably miss the real meaning.

According to the Swedish Shorinji Kempo Federation, this is the second characteristic of Shorinji Kempo.

This post really explains the concept best in my opinion: Bushido by MS: Riki Ai Fu Ni, which states: "Riki Ai Funi" is the philosophy that power (Riki) and love (Ai) are indivisible. More concretely, a person, who is powerful but does not have love, cannot control and misuse his/her power; on the other hand, a person, who has loved ones but is not powerful enough, cannot protect himself/herself nor loved ones.