Keltic -rix also meant king, most obviously in Caesar's foe Vercingetorix and that other famous Gaul, Asterix. All the way over in India, the Sanskrit word for king is rajah. Maharajah means "great king" — the first element is the same as major, much, magnify, Greek mega- and macro-, and so on. (The largest town of Tolkien's hobbits was Mickel Delving, literally the Big Dig. C.f. the underground capitol of his dwarfs — Dwarrowdelf, the Dwarfs' Dig. In general, Tolkien is a wonderful place to find words coined on OE roots.) The Sanskrit title of respect mahatma is maha-atma, great-soul.
Getting off the subject slightly, atma is an example of the widespread duality of words for "soul" and "breath", because it's related to Greek atmosphere and Latin ether. Latin spiro- is another example of this, since inspire, expire, respire, and perspire all relate to breathing (to perspire is to "breathe through" the skin), but spirit (e.g., the Holy Spirit) means soul. Inspiration has both meanings — literally, breathing in, and figuratively, breathing the soul into something, as in the story of the creation of Adam — while expiration both means breathing out and the soul leaving the body at death. The original reason for covering the mouth during a yawn or cough was the folk belief that the soul might escape from the body at such times.
A third example of the duality is Latin anima which also meant both "breath" and "soul", so magnanimous is exactly the same word as mahatma, while animal and animated have the sense of "living, breathing". Unanimous is literally "with one breath". Animosity originally meant bravery — having spirit. The change to "hatred" is by way of "animosity towards the enemy". Similarly, animadversion is "turning the mind towards" [something]. The meaning changed from "notice" to "warning" to "criticism", and in law it meant punishment, by way of "the animadversion of the judge".) If someone has equanimity, they have an "even mind". The Greek anemometer measures the wind, while the anemone waves in it. A Gaelic relative is the personal name Enid, Welsh for "soul". The opposite of "magnanimous" is pusillanimous, small-souled; for the change to "cowardly", c.f. "animosity", above. For the "small" meaning, see the section about poor Paul the pedophile.
As you may have gathered by now, there is an Indo-European me- or megh- root meaning great, seen in these mega-, major, maha-, and mickle words. Other relatives are master and mistress, majesty, maximum, magnify, mayor, Charlemagne, and the month of May (the growing season). Much originally was a synonym of "great". This is obsolete except in English place names like Much Wenlock and in Shakespeare's "much thanks", although there is an echo in phrases like "He's not much." As mentioned elsewhere, oe mo produced more and most.
Source and surge are unlikely relatives of Latin regere. They are actually the same word; the original meaning was "move quickly in a straight line". Source came to be used as the name of a fountainhead — c.f. a spring of water which "springs" out of the ground.
There are a lot of phrases in English where "straight" is used to mean "correct" — a reformed criminal has "gone straight", someone can get their "directions straight", a person who misbehaves can be advised to "straighten up and fly right" or "get their act straight", a noticeably proper person is a straight arrow, etc. Straight as the opposite of gay therefore contains a moral judgment, implying homosexuals are incorrect. To be on the "straight and narrow" is not related, though. As mentioned elsewhere, it is really "strait", so the phrase is literally "narrow and narrow".
Note the triple sense of "rule" (from Latin regula): It is an object that helps draw or measure straight lines, a guideline (the same figure of speech!) for proper behavior, and as a verb, to give orders. In fact, the first meaning of "regular" was "constrained by a rule"; it was applied to priests who had also taken a monastic vow (the regular clergy) as opposed to those who had not (the secular clergy).
The completely unrelated Greek ortho- root shows exactly the same duality of meanings, by the way. It meant both "straight" or "erect" (as in orthopedic, orthodontist, and orthogonal) and "correct", seen in orthodox.