Here's the frankly too long explaination.

I basically crammed in this drawing all my favourite motifs and all of my studies.

The tattoo on Sam's back is something I designed myself and represents Mater Dolorosa (Our Lady of Sorrows or Madonna Addolorata) with a stanza from the prayer "Stabat Mater" that goes as folows - "Through her soul, of joy bereaved, bowed with anguish, deeply grieved, now at length the sword hath passed."

Then we have Saint Michael slaughtering the Devil on Dean's arm. This is a mesh up of different stock images; usually Saint Michael stands over a serpent not a dragon and has a sword, not a halberd, those come from Saint George (Patron Saint of soldiers, so not a random choice)

Then there's the scripts. (Open the first pic)

The roman numbers on Dean's collarbone: II.V.XCIII = 2.5.83, Sam's date of birth.

The matching latin tattoos on their bicepts: The whole quote actually is "Nec tecum, nec sine te vivere possum" by Ovidius, which means "I can't live with you nor without you" and I thought of splitting it like that because I think it sums up their relationship pretty well. They can't be together, especially Sam imo (forever the runaway), because it's basically self-destructive, but being apart maybe is even worse (suicidal Dean anyone?).

A mandala with a mantra on Sam's tigh: गते गते पारगते पारसंगते बोधि स्वाहा . The Devanāgarī, for those unfamiliar, is a South Asian writing system. Here we have the mantra that closes the Heart Sutra "Gone, gone, gone to the other shore, Awakening, Svaha." (Svaha is the Hindu goddess of sacrifices; in a Buddhist context the term is used as denoument for mantras, espicially duing rites and offerings).

The characters on Dean's bicept, next to Saint Michael: 天罰. Those are technically Hanja aka Traditional Chinese characters used to write Korean (mostly because I first encountered this word in my korean phylology studies and that's how I know it), but they mean the same thing in Korean, Chinese and Japanese, "Divine Punishment", sooo - the only difference is the ponunciation. Korean: 천벌 (cheonbeol) Japanese: てんばつ (tenbatsu) Chinese: 天罚 (tiān fá).

Lastly, on Dean ribs, say hi to my bestie Hammurabi and his famous Code: here we have the Law 196 which actually even rocks know about; An eye for an eye. The full text says: "If a man has blinded the eye of another man, his eye will be blinded." Can't really offer you a transcript but if you're curious here's the transliteration: