From the D&D 5e PHB, page 177:
Constitution measures health, stamina, and vital force.
The ability description goes on to give deeper examples, but there already are three great ways to imagine a strong and dexterous character who has low constitution:
They may be in poor health. Perhaps prone to constant sickness, fainting, and bruising at the slightest bump.
They may have poor stamina. This character is capable of climbing a sheer wall or vaulting a wide chasm... once. Then they need to rest a while.
They may have poor vitality. This means a lack of vigorousness, liveliness, spunk. Perhaps this character is capable of astounding feats... but can hardly be called upon to do them without complaining or dragging their feet.
This short list represents a few fun ways to take a mechanical disadvantage and make an interesting character out of it. And of course, these three traits also can be found together.
You may be disappointed to see how Constitution might undercut your character's Strength and Dexterity - after all, what's the point of being a great athlete if you can't go the distance? But don't forget that an ability score of 6 - which imposes a -2 modifier - is no more a guarantee that your character will fail to perform than a 14 (modifier of +2) guarantees success!
Inter-Ability Score Synergy
Your character is also "lucky" enough to have a low charisma. Now, many people assume low charisma means ugly, or rude, or socially awkward in some way. But charisma is also tied to willpower in 5th edition. Page 178 of the PHB (emphasis mine):
Charisma ... includes such factors as confidence and eloquence, and it can represent a charming or commanding personality.
Also consider that a charisma saving throw is required of a creature targeted by spells such as Bane (which forces penalties upon the target) and Seeming (which can change your appearance against your will).
Therefore, a low constitution/charisma combo might suggest a character who is a bit of a wimp: low threshold for pain, easily bullied, perhaps a bit scared of being hit. That's just one suggestion, and of course you should only play a character that way to the extent that it's fun for you. But it's an example of combining scores into a deeper portrait of your wizard.